[2.0] Fixing behavior if response is `nil`

Testing without internet connection, `operation.response` is `nil` and the `failure` block is not called. We can get the URL from the `operation.request` instead of `operation.response` (as done in the `success` block, a few lines above).

I don't know if this was occurring in 1.x, so maybe we need to change this on 1.x branch too.