The MySQL backend on Django 1.8 through 1.10 has a heavy-handed

workaround for some of its SQL generation around indexes. It tries to

disable setting indexes on a ForeignKey, leaving that up to MySQL

itself, by setting db_index = False. It claims this is "temporary,"

but it never restores the values. This impacts signatures of these

fields on these versions of Django (and breaks unit tests).
This patch preserves the list of fields that would have their values

reset and then restores db_index on each of them, allowing Django to

continue to avoid those indexes without breaking any future usage of

the flags on these fields.

MySQL unit tests pass on all supported versions of Django.