Set content length in request/response bodies (#96)

### Motivation

A surprising oversight, we were never setting the `content-length`
header when sending out a known-length body.

Some transports might be already doing this, but this change makes
things more consistent.

### Modifications

Add the `content-length` header when setting a body and we know the
length from the `HTTPBody`.

### Result

More consistent experience.

### Test Plan

Adapted unit tests.

Sources/OpenAPIRuntime/Conversion/CurrencyExtensions.swift

@@ -435,8 +435,10 @@ extension Converter {
         contentType: String,
         convert: (T) throws -> HTTPBody
     ) rethrows -> HTTPBody {
+        let body = try convert(value)
         headerFields[.contentType] = contentType
-        return try convert(value)
+        if case let .known(length) = body.length { headerFields[.contentLength] = String(length) }
+        return body
     }
 
     /// Sets the provided request body and the appropriate content type.
@@ -597,8 +599,10 @@ extension Converter {
         contentType: String,
         convert: (T) throws -> HTTPBody
     ) rethrows -> HTTPBody {
+        let body = try convert(value)
         headerFields[.contentType] = contentType
-        return try convert(value)
+        if case let .known(length) = body.length { headerFields[.contentLength] = String(length) }
+        return body
     }
 
     /// Returns a decoded value for the provided path parameter.

Tests/OpenAPIRuntimeTests/Conversion/Test_Converter+Client.swift

@@ -95,7 +95,7 @@ final class Test_ClientConverterExtensions: Test_Runtime {
             contentType: "application/json"
         )
         try await XCTAssertEqualStringifiedData(body, testStructPrettyString)
-        XCTAssertEqual(headerFields, [.contentType: "application/json"])
+        XCTAssertEqual(headerFields, [.contentType: "application/json", .contentLength: "23"])
     }
 
     func test_setOptionalRequestBodyAsJSON_codable_string() async throws {
@@ -106,7 +106,7 @@ final class Test_ClientConverterExtensions: Test_Runtime {
             contentType: "application/json"
         )
         try await XCTAssertEqualStringifiedData(body, testQuotedString)
-        XCTAssertEqual(headerFields, [.contentType: "application/json"])
+        XCTAssertEqual(headerFields, [.contentType: "application/json", .contentLength: "7"])
     }
 
     //    | client | set | request body | JSON | required | setRequiredRequestBodyAsJSON |
@@ -118,7 +118,7 @@ final class Test_ClientConverterExtensions: Test_Runtime {
             contentType: "application/json"
         )
         try await XCTAssertEqualStringifiedData(body, testStructPrettyString)
-        XCTAssertEqual(headerFields, [.contentType: "application/json"])
+        XCTAssertEqual(headerFields, [.contentType: "application/json", .contentLength: "23"])
     }
 
     //    | client | set | request body | urlEncodedForm | codable | optional | setRequiredRequestBodyAsURLEncodedForm |
@@ -136,7 +136,7 @@ final class Test_ClientConverterExtensions: Test_Runtime {
         }
 
         try await XCTAssertEqualStringifiedData(body, testStructURLFormString)
-        XCTAssertEqual(headerFields, [.contentType: "application/x-www-form-urlencoded"])
+        XCTAssertEqual(headerFields, [.contentType: "application/x-www-form-urlencoded", .contentLength: "41"])
     }
 
     //    | client | set | request body | urlEncodedForm | codable | required | setRequiredRequestBodyAsURLEncodedForm |
@@ -148,7 +148,7 @@ final class Test_ClientConverterExtensions: Test_Runtime {
             contentType: "application/x-www-form-urlencoded"
         )
         try await XCTAssertEqualStringifiedData(body, testStructURLFormString)
-        XCTAssertEqual(headerFields, [.contentType: "application/x-www-form-urlencoded"])
+        XCTAssertEqual(headerFields, [.contentType: "application/x-www-form-urlencoded", .contentLength: "41"])
     }
 
     //    | client | set | request body | binary | optional | setOptionalRequestBodyAsBinary |
@@ -160,7 +160,7 @@ final class Test_ClientConverterExtensions: Test_Runtime {
             contentType: "application/octet-stream"
         )
         try await XCTAssertEqualStringifiedData(body, testString)
-        XCTAssertEqual(headerFields, [.contentType: "application/octet-stream"])
+        XCTAssertEqual(headerFields, [.contentType: "application/octet-stream", .contentLength: "5"])
     }
 
     //    | client | set | request body | binary | required | setRequiredRequestBodyAsBinary |
@@ -172,7 +172,7 @@ final class Test_ClientConverterExtensions: Test_Runtime {
             contentType: "application/octet-stream"
         )
         try await XCTAssertEqualStringifiedData(body, testString)
-        XCTAssertEqual(headerFields, [.contentType: "application/octet-stream"])
+        XCTAssertEqual(headerFields, [.contentType: "application/octet-stream", .contentLength: "5"])
     }
 
     //    | client | set | request body | multipart | required | setRequiredRequestBodyAsMultipart |

Tests/OpenAPIRuntimeTests/Conversion/Test_Converter+Server.swift

@@ -316,7 +316,7 @@ final class Test_ServerConverterExtensions: Test_Runtime {
             contentType: "application/json"
         )
         try await XCTAssertEqualStringifiedData(data, testStructPrettyString)
-        XCTAssertEqual(headers, [.contentType: "application/json"])
+        XCTAssertEqual(headers, [.contentType: "application/json", .contentLength: "23"])
     }
 
     //    | server | set | response body | binary | required | setResponseBodyAsBinary |
@@ -328,7 +328,7 @@ final class Test_ServerConverterExtensions: Test_Runtime {
             contentType: "application/octet-stream"
         )
         try await XCTAssertEqualStringifiedData(data, testString)
-        XCTAssertEqual(headers, [.contentType: "application/octet-stream"])
+        XCTAssertEqual(headers, [.contentType: "application/octet-stream", .contentLength: "5"])
     }
 
     //    | server | set | response body | multipart | required | setResponseBodyAsMultipart |