Support for URL encoded form posts.

I'm not sure if this is done yet. So far there's only support for URL encoded form data in the request body -- not the response. I've never come across an API that responds with form data. The Slack API is the only API I can think of that accepts form data, but it responds with JSON.

The other thing I'm not sure what to do about is XML. There was a `BodySerializationMethod.Xml` but I removed it because, as far as I can tell, refit doesn't support XML at all yet. There was definitely nothing referring to that enum value anyway.
We're not going to be constantly reading and writing to this collection from multiple threads, it's actually more efficient to just use `lock` in these cases