More comprehensive tests for HTTP verb endpoints (#88)

We were not explicitly testing the behavior of some HTTP verb endpoints
like /put and /patch, because they currently share an underlying handler
with /post which is thoroughly tested.

The addition of the /anything endpoint in #83 made me realize a bit more
explicit test coverage would be good, so here we're landing a bit of a
refactoring of the test suite to generate tests for all of those
endpoints.

Along the way, we also improve compatibility with the original httpbin
implementation by tricking the stdlib net/http machinery into parsing
request bodies for DELETE requests.