(PE-5653) Correctly proxy non-GET methods

Some code we brought across from the original ring-proxy seemed to be
specifically for clj-http, and it was causing POST/PUT requests to
fail. This commit removes that code and standardises on `slurp`'ing the
incoming request body before sending it to the proxied server.
Merge pull request #5 from praxxis/PE-5653_post

(PE-5653) Correctly proxy non-GET methods