Changing btoa ponyfill to improve testing

lib/helpers/btoa.js

@@ -4,12 +4,12 @@
 
 var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
 
-function InvalidCharacterError(message) {
-  this.message = message;
+function E() {
+  this.message = 'String contains an invalid character';
 }
-InvalidCharacterError.prototype = new Error;
-InvalidCharacterError.prototype.code = 5;
-InvalidCharacterError.prototype.name = 'InvalidCharacterError';
+E.prototype = new Error;
+E.prototype.code = 5;
+E.prototype.name = 'InvalidCharacterError';
 
 function btoa(input) {
   var str = String(input);
@@ -26,7 +26,7 @@ function btoa(input) {
   ) {
     charCode = str.charCodeAt(idx += 3 / 4);
     if (charCode > 0xFF) {
-      throw new InvalidCharacterError('INVALID_CHARACTER_ERR: DOM Exception 5');
+      throw new E();
     }
     block = block << 8 | charCode;
   }

test/specs/__setupBasicAuthTest.js

@@ -1,4 +1,5 @@
 var axios = require('../../index');
+var validateInvalidCharacterError = require('./__validateInvalidCharacterError');
 
 module.exports = function setupBasicAuthTest() {
   beforeEach(function () {
@@ -36,7 +37,7 @@ module.exports = function setupBasicAuthTest() {
     }).then(function(response) {
       done(new Error('Should not succeed to make a HTTP Basic auth request with non-latin1 chars in credentials.'));
     }).catch(function(error) {
-      expect(error.message).toEqual('INVALID_CHARACTER_ERR: DOM Exception 5');
+      validateInvalidCharacterError(error);
       done();
     });
   });

test/specs/__validateInvalidCharacterError.js

@@ -0,0 +1,4 @@
+module.exports = function validateInvalidCharacterError(error) {
+  expect(error instanceof Error).toEqual(true);
+  expect(/character/i.test(error.message)).toEqual(true);
+};

test/specs/helpers/btoa.spec.js

@@ -1,4 +1,5 @@
 var __btoa = require('../../../lib/helpers/btoa');
+var validateInvalidCharacterError = require('../__validateInvalidCharacterError');
 
 describe('btoa polyfill', function () {
   it('should behave the same as native window.btoa', function () {
@@ -22,6 +23,7 @@ describe('btoa polyfill', function () {
       err2 = e;
     }
 
-    expect(err1.message).toEqual(err2.message);
+    validateInvalidCharacterError(err1);
+    validateInvalidCharacterError(err2);
   });
 });