Add support for HTTP PATCH verb

Handled by the OAuth2\Client the same way as PUT. Tested and working with the [Salesforce.com REST API](http://www.salesforce.com/us/developer/docs/api_rest/) which [requires](http://www.salesforce.com/us/developer/docs/api_rest/Content/dome_update_fields.htm)  it.

See [RFC 5789](http://tools.ietf.org/html/rfc5789).