Correct Case Sensitivity for HTTP methods

Thanks for creating frisby!

I ran into a problem implementing HTTP Digest Authentication with frisbyjs that came down to the case of the HTTP methods.  The methods are case-sensitive and should be in all caps (http://www.w3.org/Protocols/HTTP/Methods.html).

This is important for HTTP digest auth because the method is used as a part of building the HA2 hash.