Refactor module tests.test_client

Drastically simplify the tests for wrapper methods such as
`nailgun.client.delete`. Make `ContentTypeIsJsonTestCase` a bit more thorough
and easy to read. Merge the contents of `ArgTestCase` in to `ClientTestCase` and
drop all references to the `_call_requests_*` methods. Drop all references to
DDT and drop it as a requirement.
Drop `nailgun.client._call_requests_*` methods

The `_call_requests_*` methods were originally created with the intent of making
tests easier to write. The idea is that those methods were small, simple pieces
of code that sat at the edge of this application and the requsts library.
However, the methods are not actually needed. The standard library tools for
mocking make it easy to directly alter how requests methods behave. Drop them.