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 check for an identical status code on lines 37 and 39. I create the `status_code` variable to prevent myself from changing code on one of those two lines, then forgetting to change code on the other of those two lines. If you think readability is a greater concern than the possible inconsistency, I'll get rid of variable `status_code`.