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

mongoengine/document.py

@@ -234,11 +234,16 @@ class Document(BaseDocument):
                     select_dict[actual_key] = doc[actual_key]
 
                 upsert = self._created
+                work = {}
+
                 if updates:
-                    collection.update(select_dict, {"$set": updates},
-                        upsert=upsert, safe=safe, **write_options)
+                    work["$set"] = updates
+
                 if removals:
-                    collection.update(select_dict, {"$unset": removals},
+                    work["$unset"] = removals
+
+                if work:
+                    collection.update(select_dict, work,
                         upsert=upsert, safe=safe, **write_options)
 
             warn_cascade = not cascade and 'cascade' not in self._meta