Implemented Response#handled_response

In Typhoeus pre-0.5.0, there was a method on request that for the handled_response would return the handled_response if it had been set otherwise would return the response

https://github.com/typhoeus/typhoeus/blob/v0.4.2/lib/typhoeus/request.rb#L185

After handled_response was returned in the Typhoeus::Response object in 0.5.4, a similiar method to the method mentioned above was not returned as well. This puts the handled_response method into Typhoeus::Response with a similar implementation.