Last argument should be handled for observable methods

Hello.

I'm very happy that `retrofit` supports `RxJava` out of the box now, but I've faced a little problem. `RequestBuilder` ignores the last argument of the method if it is observable. For example, I have the following method in my interface: `@GET ("/api/video/{id}/{token}/like") Observable<Response> like(@Path("id") long id, @Path("token") String token);`

And it fails with `retrofit.RetrofitError: java.io.IOException: Illegal character in path at index 33: http://apihost.com/api/video/604/{token}/like` when I'm trying to execute it.

My pull request fixes the problem.