refactor(helpers): optimize the logic of isAxiosError

1. add the judgment of null

lib/helpers/isAxiosError.js

@@ -1,11 +1,13 @@
 'use strict';
 
+var utils = require('./../utils');
+
 /**
  * Determines whether the payload is an error thrown by Axios
  *
  * @param {*} payload The value to test
  * @returns {boolean} True if the payload is an error thrown by Axios, otherwise false
  */
 module.exports = function isAxiosError(payload) {
-  return (typeof payload === 'object') && (payload.isAxiosError === true);
+  return utils.isObject(payload) && (payload.isAxiosError === true);
 };

test/specs/helpers/isAxiosError.spec.js

@@ -17,4 +17,9 @@ describe('helpers::isAxiosError', function () {
     expect(isAxiosError(new Error('Boom!')))
       .toBe(false);
   });
+
+  it('should return false if the error is null', function () {
+    expect(isAxiosError(null))
+      .toBe(false);
+  });
 });