Revert `APITestCase` defaulting to JSON format.

[KaczuH]

It's an attempt to fix bug reported in #205 It's been introduced in #190

I've looked into it. Changes i've introduced:

• added tests for GET requests
• corrected existing tests for POST requests (format and content_type options were used incorrectly)
• removed APITestCase.request as it's redundant. In #81 the solution should be to recommend users to set default format in tests. DRF docs

All changes are explained in commit messages.

All nox tests pass locally.