Ensure that the update actions are grouped rather than serial.

This is a performance update. When multiple properties of the same
entity have been deleted and modified, 2 calls to update the entity are
made, one {"$set": … } and another {"$unset": … }. This is 2 network
interface calls which is a performance killer (even at lan speeds).

Fixes: #210
Merge branch 'single-work-op' of https://github.com/njoyce/mongoengine into 211

Conflicts:
	mongoengine/document.py
	tests/test_document.py
Can call del Doc.attr to delete field value