Add HTTP PATCH Method

Quoting the abstract of [RFC 5789](https://tools.ietf.org/html/rfc5789):
> Several applications extending the Hypertext Transfer Protocol (HTTP)
> require a feature to do partial resource modification.  The existing
> HTTP PUT method only allows a complete replacement of a document.
> This proposal adds a new HTTP method, PATCH, to modify an existing
> HTTP resource.