improve: 为了尽可能兼容app的api，提取fetch到polyfill

src/utils/delete.js

-import fetch from 'dva/fetch';
 import _ from 'lodash';
+import { fetch } from './polyfill';
 import { encrypt } from './helper';
 import { errors } from './error';
 import { getToken } from './auth';

src/utils/polyfill.js

+/* eslint-disable */
+
+export const fetch = require('dva/fetch');

src/utils/post.js

 /* eslint-disable no-param-reassign */
-import fetch from 'dva/fetch';
 import _ from 'lodash';
+import { fetch } from './polyfill';
 import { encrypt } from './helper';
 import { checkStatus, normParams, parseObject } from './http-helper';
 import { getToken } from './auth';

src/utils/request.js

-import fetch from 'dva/fetch';
 import _ from 'lodash';
+import { fetch } from './polyfill';
 import { encrypt } from './helper';
 import { getToken } from './auth';
 import { errors } from './error';