Added support for HTTP PATCH method

The PATCH method was [published as an RFC](http://tools.ietf.org/html/rfc5789) in 2010 and is now in use by some REST APIs (e.g. [ArangoDB](https://www.arangodb.org/manuals/current/RestDocument.html)) to supplement the PUT method (although the exact semantics are often implementation-specific).

I've added a corresponding method and a test case. The behaviour is analogous to PUT.