fix; call toObject() when setting single embedded schema to doc (Fix #3535)

Author: vkarpov15

lib/document.js

@@ -503,6 +503,11 @@ Document.prototype.set = function(path, val, type, options) {
           this.set(path[key], prefix + key, constructing);
         } else if (strict) {
           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) {
+              path[key] = path[key].toObject({ virtuals: false });
+            }
             this.set(prefix + key, path[key], constructing);
           } else if (pathtype === 'nested' && path[key] instanceof Document) {
             this.set(prefix + key,