Refactor #last_request and #last_response

         use `nil or raise` convention to raise an exception.
Fixed #50 change uri to /cookie/default-path/

      post to /cookie/default-path/
      instead of /cookie/default-path (without trailing slash),
      so the default path for the newly generated cookie is
      "/cookie/default-path"
Fix bug in get '/cookie/default-path/'

    should return the request's cookies not the response's.
    And the response object does not have a #cookies method,
    this result in a 500 error.
Merge branch 'refactor-last-request-and-response'