Restrict is(redirect|error) to integers (#1117)

I used `isredirect` on a `HTTP.Stream` and instead of getting a
`MethodError` this returned `false`. This patch limits the input to
`isredirect` and `iserror` to `Integer`s.

src/Messages.jl

@@ -235,8 +235,8 @@ issafe(method) = method in ["GET", "HEAD", "OPTIONS", "TRACE"]
 Does this `Response` have an error status?
 """
 iserror(r::Response) = iserror(r.status)
-iserror(status) = status != 0 && status != 100 && status != 101 &&
-                 (status < 200 || status >= 300) && !isredirect(status)
+iserror(status::Integer) = status != 0 && status != 100 && status != 101 &&
+                          (status < 200 || status >= 300) && !isredirect(status)
 
 """
     isredirect(::Response)
@@ -245,7 +245,7 @@ Does this `Response` have a redirect status?
 """
 isredirect(r::Response) = isredirect(r.status)
 isredirect(r::Request) = allow_redirects(r) && !redirectlimitreached(r)
-isredirect(status) = status in (301, 302, 303, 307, 308)
+isredirect(status::Integer) = status in (301, 302, 303, 307, 308)
 
 # whether the redirect limit has been reached for a given request
 # set in the RedirectRequest layer once the limit is reached