Add support for HTTP PATCH in RestHelper

[ricsirigu]

Mailing List thread: https://groups.google.com/forum/#!topic/liftweb/4ByUPd1TMkc

Lift doesn't provide support for the http Patch method out of the box.
Even if it could be implemented inspecting the method name of the underlying Http Req, it should be cool to have direct support of it in RestHelper