Fixed EmailField so can add extra validation

(MongoEngine/mongoengine#173, MongoEngine/mongoengine#174, MongoEngine/mongoengine#187)

mongoengine/queryset.py

@@ -929,7 +929,7 @@ class QuerySet(object):
             if not isinstance(doc, self._document):
                 msg = "Some documents inserted aren't instances of %s" % str(self._document)
                 raise OperationError(msg)
-            if doc.pk:
+            if doc.pk and not doc._created:
                 msg = "Some documents have ObjectIds use doc.update() instead"
                 raise OperationError(msg)
             raw.append(doc.to_mongo())