Refactor creation of the response object

`Proxy#perform_request` has gotten pretty big, specifically the `target_response` initialization. I've refactored that part a little, removing duplication.

The only meaningful change was renaming `HttpStreamingResponse#status` to `#code`, so that its API matches `Net::HTTP#code`. I've added an alias so that `#status` keeps woking, in case someone's using it externally.