lib/twitter/client.rb: Fixing issue with wrong signature being generated when multipart data is posted.

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.
Adding tests to check if the client generates correct OAuth signature in simple cases
Merge pull request #412 from NirgunaSarvajna/master

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