Add test for content type header

[Marishka17]

Motivation and context

How has this been tested?

Checklist

• I submit my changes into the develop branch
  - [ ] I have added a description of my changes into CHANGELOG file
  - [ ] I have updated the documentation accordingly
• I have added tests to cover my changes
  - [ ] I have linked related issues (read github docs)
  - [ ] I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
  - [ ] I have updated the license header for each file (see an example below)

# Copyright (C) 2022 Intel Corporation
#
# SPDX-License-Identifier: MIT

[nmanovic]

@Marishka17 , could we extend https://github.com/openvinotoolkit/cvat/tree/develop/tests/rest_api? I really don't want to touch old tests anymore if they are working.

[nmanovic]

@Marishka17 , LGTM. I would rename test_data.py to test_tasks.py (or test_0010_tasks.py if we decide to keep numbers in the file name).

[nmanovic]

@kirill-sizov , please look and advise.

[sizov-kirill]

We try to avoid using hardcoded data (usernames, ids, etc.) in tests.
But even though this test assumes that tasks with ids 1, 5, 6 will never change Content-Type and that user1 will always see them, I think this is a reasonable limitation because it is difficult to change this information (except for example by deleting a task).

For sure it's easy to avoid this limitation with creating another one JSON file headers.json, but I suggest to delay this idea till it will be really needed.

[nmanovic]

LGTM