Make .oauth lazily add headers to request

[hoffrocket]

it currently signs and adds the Authorization header at the point at which it's called, so further changes to the request are not part of the signature and that will cause oauth failures.

[tpravin]

To add to this, like oauth_callback=oob is not part of the Authorization Header hence I am not able to generate a request_token because of a rejection from oauth provider saying "Missing callback".

[brodin]

Any updates to this @hoffrocket ? :)