This fixes some test failures on MySQL 5.7+.

Partially reverted refs #25532 to fix a regression in Django 1.10.
This reintroduces a crash for disabled forms.JSONField (refs #26949),
however, that issue is also present on Django 1.9.

Thanks Ryan Schave for the test.

Unused as of 2b039d96.

Thanks Simon Charette for help with the patch.

Thanks Trac alias karyon for the report and Tim for the review.

This is a security fix.