Fix invalid JSON during morango sync

[bjester]

Summary

Updates the content negotiation class to use default behavior for morango.

Reviewer guidance

• Needs a GZIP enabled peer
• Pull a facility first, make changes, then try to sync with the peer you pulled from (it should push)

References

…

---

Contributor Checklist

PR process:

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Testing:

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

Reviewer Checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[bjester]

@jredrejo Could you verify that removing the select_parser override works as expected with whatever prompted your multipart additions?

[codecov]

Codecov Report

Merging #7226 into release-v0.14.x will decrease coverage by 0.07%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
kolibri/core/negotiation.py	100.00% <100.00%> (+16.66%)	⬆️
kolibri/utils/logger.py	82.50% <0.00%> (-12.50%)	⬇️
kolibri/plugins/app/utils.py	51.51% <0.00%> (-10.99%)	⬇️
.../core/deviceadmin/management/commands/dbrestore.py	76.56% <0.00%> (ø)
...oach/assets/src/views/plan/PlanQuizPreviewPage.vue	0.00% <0.00%> (ø)
.../src/views/plan/LessonContentPreviewPage/index.vue	0.00% <0.00%> (ø)
...c/views/plan/PlanLessonSelectionContentPreview.vue	0.00% <0.00%> (ø)
...ws/plan/LessonContentPreviewPage/SelectOptions.vue
...gins/device/assets/src/views/HeaderWithOptions.vue	0.00% <0.00%> (ø)