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 is text/plain. Add a test which directly targets this bug.
I was about to ask in your other old code as well. Do we really need this line or just use httplib.CREATED in line 39 instead?