improve asString performance (#592)

* improve asString performance

* Update lib/serializer.js

Co-authored-by: Rafael Gonzaga <rafael.nunu@hotmail.com>

* fix lint issue

Co-authored-by: Rafael Gonzaga <rafael.nunu@hotmail.com>

Author: Uzlopak

lib/serializer.js

@@ -98,23 +98,24 @@ module.exports = class Serializer {
   }
 
   asString (str) {
-    const quotes = '"'
-    if (str instanceof Date) {
-      return quotes + str.toISOString() + quotes
-    } else if (str === null) {
-      return '""'
-    } else if (str instanceof RegExp) {
-      str = str.source
-    } else if (typeof str !== 'string') {
-      str = str.toString()
+    if (typeof str !== 'string') {
+      if (str === null) {
+        return '""'
+      }
+      if (str instanceof Date) {
+        return '"' + str.toISOString() + '"'
+      }
+      if (str instanceof RegExp) {
+        str = str.source
+      } else {
+        str = str.toString()
+      }
     }
 
     // Fast escape chars check
     if (!STR_ESCAPE.test(str)) {
-      return quotes + str + quotes
-    }
-
-    if (str.length < 42) {
+      return '"' + str + '"'
+    } else if (str.length < 42) {
       return this.asStringSmall(str)
     } else {
       return JSON.stringify(str)