Fixed #2170 -- "exact" lookups in MySQL are now case-sensitive (the same as other backends).

This is a backwards incompatible change if you were relying on 'exact' being
case-insensitive. For that, you should be using 'iexact'.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7798 bcc190cf-cafb-0310-a4f2-bffc1f526a37