Refs #2061. mungedb-aggregate cannot group on _id:null .

lib/pipeline/documentSources/GroupDocumentSource.js

@@ -78,6 +78,7 @@ var GroupDocumentSource = module.exports = (function(){
 
                         idSet = true;
 					}else{
+						//! @todo: _getTypeStr() breaks when trying to group on _id:null.
 						var typeStr = group._getTypeStr(groupField);
 						switch(typeStr){
 							case "number":