allow models to deserialize an azure.core.rest.HttpResponse

Basically on line `1538` of serialization.py, set to internal response if it exists. This way, we can just go ahead and deal with the internal `azure.core.pipeline.transport.HttpResponse` instead of our client-facing `azure.core.rest.HttpRequest` wrapper

```
try:
    raw_data = raw_data._internal_response
except AttributeError:
    pass
```

Add tests that this works