perf: couple of small micro-optimizations re: #13191

Author: vkarpov15

lib/document.js

@@ -1130,8 +1130,8 @@ Document.prototype.$set = function $set(path, val, type, options) {
       }
 
       if (utils.isNonBuiltinObject(valForKey) && pathtype === 'nested') {
-        this.$set(prefix + key, path[key], constructing, Object.assign({}, options, { _skipMarkModified: true }));
-        $applyDefaultsToNested(this.$get(prefix + key), prefix + key, this);
+        this.$set(pathName, path[key], constructing, Object.assign({}, options, { _skipMarkModified: true }));
+        $applyDefaultsToNested(this.$get(oathName), pathName, this);
         continue;
       } else if (strict) {
         // Don't overwrite defaults with undefined keys (gh-3981) (gh-9039)
@@ -1146,9 +1146,9 @@ Document.prototype.$set = function $set(path, val, type, options) {
 
         if (pathtype === 'real' || pathtype === 'virtual') {
           const p = path[key];
-          this.$set(prefix + key, p, constructing, options);
+          this.$set(pathName, p, constructing, options);
         } else if (pathtype === 'nested' && path[key] instanceof Document) {
-          this.$set(prefix + key,
+          this.$set(pathName,
             path[key].toObject({ transform: false }), constructing, options);
         } else if (strict === 'throw') {
           if (pathtype === 'nested') {
@@ -1158,7 +1158,7 @@ Document.prototype.$set = function $set(path, val, type, options) {
           }
         }
       } else if (path[key] !== void 0) {
-        this.$set(prefix + key, path[key], constructing, options);
+        this.$set(pathName, path[key], constructing, options);
       }
     }