Make last_response available on save()

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries @jleclanche

Fixes #393.

When calling save(), convert_to_stripe_object will call refresh_from with a StripeObject instance that already has the _last_response attribute set. However refresh_from only set the the _last_response attribute from an explicit last_response argument and did not check whether the instance already had a _last_response attribute like it does for other memorized attributes (e.g. stripe_account). This PR fixes that, and adds a bunch of tests to ensure that last_response is available on instances returned by all common API operations.

While fixing this, I also noticed a regression introduced by #371: the OAuth.token and OAuth.deauthorize methods used to directly return a dict with the JSON values returned by connect.stripe.com. After #371, those methods instead returned a StripeResponse instance. I've fixed it so that they return a dict again.

[jleclanche]

Yep, looks good.

[brandur-stripe]

Woo, great eye Olivier, and thanks for adding those additional tests/asserts!

LGTM.