Use :request-method key to pull out HTTP method

We used the wrong key, :method, to lookup the HTTP request method from
the ring request. This forced callers to remap :request-method ->
:method in their ring requests, because we were asserting that :method
was present in the request.

We now look for :request-method. We also turn the keyword to an upper
case string in http.clj.

Fixes #2