Support for Nullable and Char (#174)

* Support for `Nullable`

Added `_writejson` method for `Nullable` values. Maps to JSON `null` if
value is null.

* trying to fix test fail on 0.4

* Added  conversion

* Added more tests and converted `Char` handling to use `lower`

src/JSON.jl

@@ -56,6 +56,7 @@ if VERSION < v"0.5.0-dev+2396"
     lower(f::Function) = "function at $(f.fptr)"
 end
 
+lower(c::Char) = string(c)
 lower(d::DataType) = string(d)
 lower(m::Module) = throw(ArgumentError("cannot serialize Module $m as JSON"))
 
@@ -167,6 +168,14 @@ function _writejson(io::IO, state::State, n::Void)
     Base.print(io, "null")
 end
 
+function _writejson(io::IO, state::State, a::Nullable)
+    if isnull(a)
+        Base.print(io, "null")
+    else
+        _writejson(io, state, get(a))
+    end
+end
+
 function _writejson(io::IO, state::State, a::Associative)
     if length(a) == 0
         Base.print(io, "{}")

test/runtests.jl

@@ -251,6 +251,17 @@ end
 @test sprint(JSON.print, [NaN]) == "[null]"
 @test sprint(JSON.print, [Inf]) == "[null]"
 
+# check Nullables are printed correctly
+@test sprint(JSON.print, [Nullable()]) == "[null]"
+@test sprint(JSON.print, [Nullable{Int64}()]) == "[null]"
+@test sprint(JSON.print, [Nullable{Int64}(Int64(1))]) == "[1]"
+
+# check Chars
+@test json('a') == "\"a\""
+@test json('\\') == "\"\\\\\""
+@test json('\n') == "\"\\n\""
+@test json('🍩') =="\"🍩\""
+
 # check for issue #163
 @test Float32(JSON.parse(json(2.1f-8))) == 2.1f-8