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
fix response code in unprocessable_entity comment, fixes #409
Restore compatibility with Ruby 1.8.7

The Base64 module does not have a strict_encode64 method in Ruby 1.8.
Remove undocumented methods, retired in the APIpocalypse

See: https://dev.twitter.com/blog/api-v1-retirement-final-dates
Merge pull request #412 from NirgunaSarvajna/master

 Fixing issue with wrong signature being generated when multipart data is posted.
Fix typo in specs [ci skip]
Merge pull request #411 from mustafaturan/master

Added Twitter::SearchResults#refresh_url
Cleanup #411
Prepare for version 4.8.0 release
Add Twitter::API::Undocumented#tweet_count
Fix documentation for Twitter::API::Tweets#retweeters_ids [ci skip]
Refactor common code from Twitter::SearchResults