fix update_fields in handler

[jerch]

Fixes #87.

@stephrdev I was not able to repro #87. Can you give more insights how this was triggered in your code? For all I tried, kwargs['update_fields'] was either None or a frozenset. Which Django version are you using?

[coveralls]

Pull Request Test Coverage Report for Build 1688694270

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.503%

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Totals
Change from base Build 1639501652:	0.0%
Covered Lines:	1171
Relevant Lines:	1198

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💛 - Coveralls

[stephrdev]

I had this issue together with another library (admin-sortable2), the signal is triggered here: https://github.com/jrief/django-admin-sortable2/blob/master/adminsortable2/admin.py#L341

Your fix looks great and should solve the issue!

[jerch]

@stephrdev Thx for checking, guess I'll do a patch release for this.

The usage in django-admin-sortable2 looks wrong to me following the official django docs, which states in https://docs.djangoproject.com/en/4.0/ref/signals/#pre-save:

update_fields The set of fields to update as passed to Model.save(), or None if update_fields wasn’t passed to save().

Note the word set. I also checked the django repo sources and the internals always emit a set like type if it contains anything. Maybe this is worth a bug report in django-admin-sortable2.

[stephrdev]

Thanks for merging and releasing!