fix; issue with fix for #3535

Author: vkarpov15

lib/document.js

@@ -505,7 +505,8 @@ Document.prototype.set = function(path, val, type, options) {
           if ('real' === pathtype || 'virtual' === pathtype) {
             // Check for setting single embedded schema to document (gh-3535)
             if (this.schema.paths[pathName] &&
-                this.schema.paths[pathName].$isSingleNested) {
+                this.schema.paths[pathName].$isSingleNested &&
+                path[key] instanceof Document) {
               path[key] = path[key].toObject({ virtuals: false });
             }
             this.set(prefix + key, path[key], constructing);