fix: skip adding a toJSON method to errors that already have one (#61)

Author: KyleRoss

lib/LogMessage.js

@@ -229,6 +229,8 @@ class LogMessage {
    * @returns {Error}  The original error stubbed with a toJSON() method.
    */
   static stubError(err) {
+    if(typeof err.toJSON === 'function') return err;
+
     err.toJSON = function () {
       const keys = [
         'name',
@@ -239,9 +241,9 @@ class LogMessage {
       return keys.reduce((obj, key) => {
         if(key in err) {
           const val = err[key];
-          if(typeof val !== 'function') {
-            obj[key] = val;
-          }
+
+          if(typeof val === 'function') return obj;
+          obj[key] = val;
         }
 
         return obj;