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

[iscai-msft]

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