Fixing issue with wrong signature being generated when multipart data is posted.

The recent changes released in 4.7.0 broke at least **update_with_media** API method.

Any multipart request shouldn't have it's POST parameters included while generating the OAuth signature.

The `request` method on Twitter::Client gets signature_params but
doesn't pass that along to `request_setup` method breaking the signature
on multipart posts.

This fix adds the missing extra param to `request_setup` and uses it
when generating the OAuth signature.