Add http PATCH method support to rest controllers.

When issuing a PATCH request a resource should be "patched", that is updated only with the properties submitted maintaining the rest of its properties. In contrast to POST requests, the later update the whole resource regardless of any data already there.
introduce a failing test

This failing test demonstrates one of the things that is wrong with the code currently included in the PR at https://github.com/grails/grails-core/pull/495.