[mongodb] Use frozenset instead of set.  Fixes #159
[mongodb] Use frozenset instead of set.  Fixes #159

tkanemoto/kombu,alex/kombu,iris-edu-int/kombu,numb3r3/kombu,daevaorn/kombu,andresriancho/kombu,mathom/kombu,cce/kombu,daevaorn/kombu,andresriancho/kombu,romank0/kombu,jindongh/kombu,mathom/kombu,cce/kombu,iris-edu-int/kombu,alex/kombu,urbn/kombu,romank0/kombu,ZoranPavlovic/kombu,celery/kombu,numb3r3/kombu,depop/kombu,xujun10110/kombu,tkanemoto/kombu,mverrilli/kombu,depop/kombu,ZoranPavlovic/kombu,Elastica/kombu,jindongh/kombu,Elastica/kombu,bmbouter/kombu,bmbouter/kombu,mverrilli/kombu,xujun10110/kombu