Changed "exact" matches in MySQL to use the database's native collation.

This effectively reverses the change in [7798]. It was proving too difficult to
successfully manage all the side effects here and provide a satisfactory
solution for everybody. Many thanks to arne, Martin von Löwis and, particular,
Karen Tracey, for doing a lot of research and proto-patches here to establish what was possible and practical.

This is backwards incompatible if you were relying on the behaviour after
[7798]. The docs have been updated to indicate the solution.

Refs #2170, #7789, #8102.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8319 bcc190cf-cafb-0310-a4f2-bffc1f526a37