fix(model): increment version when $set-ing it in a save() that requires a version bump

Fix #5779

Author: vkarpov15

lib/model.js

@@ -652,8 +652,13 @@ Model.prototype.$__version = function(where, delta) {
   }
 
   if (VERSION_INC === (VERSION_INC & this.$__.version)) {
-    if (!delta.$set || typeof delta.$set[key] === 'undefined') {
-      delta.$inc || (delta.$inc = {});
+    if (get(delta.$set, key, null) != null) {
+      // Version key is getting set, means we'll increment the doc's version
+      // after a successful save, so we should set the incremented version so
+      // future saves don't fail (gh-5779)
+      ++delta.$set[key];
+    } else {
+      delta.$inc = delta.$inc || {};
       delta.$inc[key] = 1;
     }
   }