mgr/dashboard: backend api tests: tasks.mgr.dashboard.test_osd.OsdTest failures

mgr/dashboard: backend api tests: tasks.mgr.dashboard.test_osd.OsdTest failures

- Fix bug in Dashboard QA unit test framework. Don't set the application type header manually, this is done by the requests library if required.
- Enhance QA unit test helper: Print the response of the API request if it fails. This should help to identify the problem more easily.
- Fix bug in the OSD controller. A parameter needs to be converted to integer.
- Take care that the params of the request object are not modified.

The issue was introduced by PR https://github.com/ceph/ceph/pull/24475. The CherryPy json_in plugin disclosed the errorneous unit test helper implementation.

Fixes: https://tracker.ceph.com/issues/36708
Signed-off-by: Volker Theile <vtheile@suse.com>