Add test targeting bugzilla bug 1116043

By default, the API client makes calls to the Foreman API with a content-type of
application/json. BZ bug 1116043 describes an issue that occurs if the
content-type if text/plain. Add a test which directly targets this bug.
Merge pull request #1060 from Ichimonji10/bugzilla

Add test targeting bugzilla bug 1116043
Improve HTTP PUT API tests, reorganize code

There are API tests that issue HTTP PUT requests and check the status code
returned by those HTTP requests. However, there are not tests which check
whether the just-updated entities were correctly updated. Add tests of this kind.

Reorganize code. Move methods `test_post_and_get` and `test_put_and_get` into
their own test case. This is done so that the `longMessage` class variable can
be used. This variable allows good quality error messages to be written while
using a minimum of code.

Update the docstring for class `EntityTestCase`.

Add a new assertion to method `test_post_status_code` and update its docstring
accordingly.
Remove code for unpacking server responses

Formerly, some server responses were in this format:

    {'entity_name': {...}}

And some server responses were in this format:

    {...}

Bugzilla bug 1087372 deals with this issue. Remove the code that allowed the
test client to deal with either type of response, as only the latter type of
response is now expected.