Figure out the actual set of supported HTTP verbs

[julik]

While working on https://github.com/julik/faraday-patron I found that the new tests for Faraday adapters also test the TRACE HTTP method - which, go figure, exists. There is also a number of more esoteric WebDAV methods (the PROPFINDs and the PROPPATCHes and the like). Given that strictly speaking the method/verb is just.. a bare string in an HTTP request we should support any verb that libcurl is willing to accept, and not a narrow subset of them.