Fixed #10646: `cache.incr()` and `decr()` now fail consistantly under...

Fixed #10646: `cache.incr()` and `decr()` now fail consistantly under python-memcache and cmemcache.

Though it looks like this commit has no tests that's not so: this is tested for in `regressiontests/cache/tests.py` around like 183; these tests currently fail if ran against cmemcache.

Thanks, andrewfong.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11855 bcc190cf-cafb-0310-a4f2-bffc1f526a37