populate; depopulate populated single paths

also fixes legacy issue where single populated path
was markedModified causing populated document to
overwrite _id.

relates to #570

lib/document.js

@@ -1417,6 +1417,11 @@ Document.prototype._doQueue = function () {
  */
 
 Document.prototype.toObject = function (options) {
+  // check for populated paths that we set to a document
+  if (options && options.convertToId) {
+    return clone(this._id, options);
+  }
+
   // When internally saving this document we always pass options,
   // bypassing the custom schema options.
   if (!(options && 'Object' == options.constructor.name)) {

lib/model.js

@@ -353,7 +353,7 @@ Model.prototype._delta = function _delta () {
       operand(self, where, delta, data, value);
 
     } else {
-      value = utils.clone(value);
+      value = utils.clone(value, { convertToId: 1 });
       operand(self, where, delta, data, value);
     }
   }

test/model.populate.test.js

@@ -2146,6 +2146,7 @@ describe('model: populate:', function(){
                 assert.equal(doc.fans[5], user7.id);
                 assert.equal(doc.fans[6], null);
                 assert.equal(doc.fans[7], String(_id));
+                assert.equal(String(doc._creator), creator._id);
                 done();
               })
             })