Add Query.get_resource_data

This new method returns the raw dictionary response
from the APIRequestor. This is used by the `get` method
to initialize the `Resource` object.

This improves testability, encapsulation, and provides a
public api for overriding. For example, previous test mocking
might override `gapipy.request.APIRequestor._request`
to return known data/dict, but this requires low level
knowledge of private behaviour and further does not allow
working around caching. New mocks can now override
`gapipy.query.Query.get_resource_data` to better control the
desired reponse.