First class XML/YAML content type support

[zacharyburkett]

Many parts of the code currently assume a content type of application/json while others are more agnostic. Certain request types aren't generated unless the content type is application/json, expecting the user to implement such types themselves.

We should find all instances of assumptions of JSON and improve the variety of content types that can be used in generation.

There are many common content types, so maybe we should consider even more.

Note: This should apply specifically to the server code, as the client is being completely remade with this in mind already.