ENH: easy access to raw response

[bsipocz]

In recent debugging sessions it became painful that the multiple inheritances and layered methods makes e.g. accessing the raw response more difficult than necessary.

So, while a full refactoring and simplification could be a huge project, giving access to the raw response could be a nice and reasonably quick enhancement.

cc @andamian and @tomdonaldson

[bsipocz]

(in astroquery we also have get_query_payload, but here I agree that raw response would be the most useful as the queries themselves are following the standard (which is not true at all at the reverse engineered webforms in astroquery)).