[1.2.X] Fixed #13967 -- MySQL spatial backend now respects when...

[1.2.X] Fixed #13967 -- MySQL spatial backend now respects when `spatial_index=False`.  Thanks, Simon Law, for bug report and patch.

Backport of r13443 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13444 bcc190cf-cafb-0310-a4f2-bffc1f526a37