fix multivalued param dict handling in to_url, fix oauth_body_hash sending for GET requests

[tylerwilliams]

These patches fix multivalued param handling for the case when you pass a dictionary with iterable values into a request and then call to_url. Previously this would encode the iterable value as a single string instead of encoding each value separately.

This also fixes the library to not send the "oauth_body_hash" parameter on GET or HEAD requests, as defined in the spec.