Improve test for updating repository content type

A repository's content type cannot be updated. Make a test which updates a
repository's content type explicitly check to make sure that its content type is
not updated.

    $ nosetests tests/foreman/api/test_repository.py -m test_update
    .............
    ----------------------------------------------------------------------
    Ran 13 tests in 25.713s

    OK

This should have been implemented in fb5f58094d7b64f7bd355ce5bd1c844f5766ab0b.