Fixed deadlock when serializing content

[stevewgh]

This fixes an issue that was discovered after pull request #571 was merged to master (MyGet version 4.6.66).

A request which contains a body that should be serialized (and is not a multi-part request) will deadlock in the PushStreamContent at:

refit/Refit/PushStreamContent.cs

Line 142 in 3fce69b

await serializeToStreamTask.Task;

The PushStreamContent content type provides a stream to write to, but in order for it to complete the request it blocks until the stream is closed. The original implementation did this implicitly because a StreamWriter was used within a using block:

refit/Refit/RequestBuilderImplementation.cs

Lines 474 to 481 in e2e7975

	ret.Content = new PushStreamContent((stream, _, __) =>
	{
	using (var writer = new JsonTextWriter(new StreamWriter(stream)))
	{
	serializer.Serialize(writer, param);
	}
	},
	new MediaTypeHeaderValue("application/json") { CharSet = "utf-8" });

I've added additional tests to make sure that the request doesn't block and it also checks the other flow through the RequestBuilderImplementation class where the request is not a multi-part request. I also fixed a typo in existing tests where a wrong exception was thrown.