HTTP: updating query and response after authentication failure

I noticed that when making a request with wrong credentials both the `Query` object returned and the `Response` object yielded to the block have the `status_code` set to nil. There's actually no easy way of knowing why the request is failing.

Im not 100% sure that this is correct behaviour, but I believe it would be convenient to set the response status to `401` unauthorized request. 

Im very new at ios and rubymotion, so the way Im fixing this might not be the best. Comments and suggestions are more than welcome.

**Implementation note**
Im calling the method `Query#connection:didReceiveResponse` from `Query#connection:didReceiveAuthenticationChallenge` and Im not sure if it's a good practice to do this, since it's a callback method and I dont know if we are supposed to use it.