stop constructing a request object in this setter

Since we just go through the normal test harness that sets up a request
for us, we don't need to do this anymore.

actionpack/lib/action_controller/metal/testing.rb

@@ -4,7 +4,6 @@ module Testing
 
     # TODO : Rewrite tests using controller.headers= to use Rack env
     def headers=(new_headers)
-      @_response ||= ActionDispatch::Response.new
       @_response.headers.replace(new_headers)
     end