Do not accept PUN/GEM methods as PUT/GET.

* Encountering them returns an error, `HPE_INVALID_METHOD`
* Tests have been added.

There's not a clear distinction between what erroneous methods should trigger `HPE_INVALID_METHOD` vs. `HPE_UNKNOWN` (there are extant tests for `HPE_UNKNOWN` against methods like "C*****"), advice would be appreciated. Tested with `make test` on OSX 10.7.5.

This fixes joyent/node#6078.