RFC: Request method whitelist and defaultCodec

I'd like to build an JSON RPC-style API but with some relaxed semantics:

- It should work with any HTTP request method (e.g. PUT/DELETE, not just POST).
- `content-type` is optional (e.g. assume `application/json` if not provided).

To that effect, I'd like to propose adding a couple of fields to the Server struct as included in this PR. This PR is not complete, as there is no public API for manipulating these fields yet. I'd love to get some feedback/ideas and a sense of whether this is something you'd be interested in merging.

P.S. Another thing I'm exploring is allowing params to be passed through the HTTP request encoding, rather than as JSON in the body. This would allow for GET requests, and easier cross-site javascript client-side requests.