With responses as a test dependency we can test methods that make
requests without hitting remote API's. This should help a lot with the
test coverage of service modules.

- self.target is referenced in ActiveCollab2Client but undefined.
- "ticket" is not a valid type to pass to get_default_description

This is more readable, yields more readable errors, and ensures that
keys aren't missed.

ServiceClient.json_response's signature takes a url parameter as well.

This ensures that certain test methods are implemented for each service.
In addition, it provides a reasonable place to document each test.

This makes the test_issues methods a bit easier to read.