Remove content-type from GET requests

[ryanackley]

The content type is unsettable from oauth.get requests so it ends up being "application/x-www-form-urlencoded". I'm trying to access a rest endpoint and this content-type is causing a 415 error "Unsupported Media Type".

[leonkyr]

when this will be merged? +1

[mikeblakeney]

For anyone else finding this, I ended up just using _performSecureRequest directly.

consumer._performSecureRequest( token, secret, "GET", url, "", null, contentType, callback)

relates to #281, #182, #183, #331, #333, #136, #284