Fixed #10438 -- Fixed MySQL backend behaviour for UPDATE behaviour.

We need to know the number of rows that are matched by an UPDATE query,
not just the number of rows that are changed. In the relatively unlikely
event that somebody was using Django's cursor proxy and relying on the
previous behaviour, well, that isn't the case any longer. We need to
this version.

Thanks to Daniel Tang for pointing out the solution here.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10532 bcc190cf-cafb-0310-a4f2-bffc1f526a37