Remove btoa polyfill tests

test/specs/basicAuth.spec.js

-describe('basicAuth without btoa polyfill', function () {
+describe('basicAuth', function () {
   setupBasicAuthTest();
 });

test/specs/basicAuthWithPolyfill.spec.js

-var window_btoa;
-
-describe('basicAuth with btoa polyfill', function () {
-  beforeAll(function() {
-    window_btoa = window.btoa;
-    window.btoa = undefined;
-  });
-
-  afterAll(function() {
-    window.btoa = window_btoa;
-    window_btoa = undefined;
-  });
-
-  it('should not have native window.btoa', function () {
-    expect(window.btoa).toEqual(undefined);
-  });
-
-  setupBasicAuthTest();
-});
-

test/specs/helpers/btoa.spec.js

-var __btoa = require('../../../lib/helpers/btoa');
-
-describe('btoa polyfill', function () {
-  it('should behave the same as native window.btoa', function () {
-    // btoa doesn't exist in IE8/9
-    if (isOldIE && typeof Int8Array === 'undefined') {
-      return;
-    }
-
-    var data = 'Hello, world';
-    expect(__btoa(data)).toEqual(window.btoa(data));
-  });
-
-  it('should throw an error if char is out of range 0xFF', function () {
-    var err;
-    var data = 'I ♡ Unicode!';
-    
-    try {
-      __btoa(data);
-    } catch (e) {
-      err = e;
-    }
-
-    validateInvalidCharacterError(err);
-  });
-});