Assign headers properly to prevent 500 when database is locked

[rtibbles]

Summary

• To preempt issues from syncing happening while learners are engaging with content, a middleware was implemented to prompt retries when a database locked error was encountered during progress tracking.
• Unfortunately, @rtibbles failed to set the headers properly, and tried to pass them as a kwarg to the Django response object, rather than setting them via attribute assignment after instantiation.
• This resolves that and assigns headers to response object properly to prevent an exception during exception handling.

References

Fixes #8866

Reviewer guidance

Run kolibri as a learner only device, engage with content while syncing is happening. See a 503, but that Kolibri eventually does make a successful request.

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Testing checklist

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

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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

[radinamatic]

I repeated all the scenarios that used to trigger 500 in the past few days, but had no luck in seeing it again 💯

[radinamatic]

Looks good, ! 👍🏽