Consider multipart/form-data encoding when signing request

Parameters being sent that are not the oauth_* named parameters do not become part of the OAuth signature base string when the HTTP method being used is POST but the post body is not of the "Content-Type: x-www-form-urlencoded" variety.

Consider multipart/form-data encoding when retrieving parameters to encode:
- Fix indentation in _getSignableParametersAsQueryString method
- Remove comment that states that a query string is returned
- Change return type to array in docblock
- Remove unnecessary code that created the query string which
  is not used.
- Add code to not include POST parameters when encoding is
  multipart/form-data

Problem popped up when I tried to send images to the twitter API. The encoding has to be multipart/form-data and the API didn't validate my credentials because they were simply wrong for that case.