perf(document): avoid cloning options using spread operator for perf reasons

Re: #14394

Author: vkarpov15

lib/document.js

@@ -3855,7 +3855,11 @@ Document.prototype.$toObject = function(options, json) {
   }
 
   // merge default options with input options.
-  options = { ...defaultOptions, ...options };
+  for (const key of Object.keys(defaultOptions)) {
+    if (options[key] == null) {
+      options[key] = defaultOptions[key];
+    }
+  }
   options._isNested = true;
   options.json = json;
   options.minimize = _minimize;