Fixed #15169 -- Corrected handling of model boolean fields in MySQL spatial backend.

Thanks goes to zmsmith and others for reporting the issue and to Justin Bronn for the fix.

Refs #7190, r12578, r12900, #13293, r12939.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17588 bcc190cf-cafb-0310-a4f2-bffc1f526a37