IGNORE_CLUSTER_ERRORS incorrectly being sent to pylibmc

[a-abella]

   File "/usr/local/lib/python2.7/dist-packages/django_elasticache/memcached.py", line 100, in _cache
     client.behaviors = self._options.get('behaviors', self._options)
   File "/usr/local/lib/python2.7/dist-packages/pylibmc/client.py", line 161, in set_behaviors
     raise ValueError("unknown behavior names: %s" % (names,))
 ValueError: unknown behavior names: IGNORE_CLUSTER_ERRORS

Entire self._options is sent to pylibmc despite django-elasticache supporting OPTIONS parameters that are incompatible with pylibmc.