refactor: replace `post` function in `api/HTTP.js` with `gbkFetch` function to support both POST and GET

BREAKING CHANGE: getting decoded gbk post data has changed.

Before:

import {post} from 'api/HTTP';
post('http://foo.bar', {header: {}, body: 'foo'});

After:

import {bgkFetch} from 'api/HTTP';
gbkFetch('POST', 'http://foo.bar', {header: {}, body: 'foo'});

Since fetching data and decoding them from gbk by GET or POST method is barely the same, it's no need to make them two functions.

src/api/HTTP.js

@@ -1,7 +1,7 @@
 import iconv from 'iconv-lite';
 import {Buffer} from 'buffer';
 
-function post(url, data) {
+function gbkFetch(method, url, data) {
   return new Promise(function (resolve, reject) {
     const request = new XMLHttpRequest();
 
@@ -15,7 +15,7 @@ function post(url, data) {
     request.onerror = () => reject(new Error(request.statusText));
     request.responseType = 'arraybuffer';
 
-    request.open('POST', url);
+    request.open(method, url);
     for (let headerKey in data.headers) {
       request.setRequestHeader(headerKey, data.headers[headerKey]);
     }
@@ -23,4 +23,4 @@ function post(url, data) {
   });
 }
 
-export {post};
+export {gbkFetch};

src/component/YooLogin.js

@@ -1,7 +1,7 @@
 import React, {Component} from 'react';
 import YooLoginUI from '../ui/YooLoginUI';
 import CookieManager from '@react-native-community/cookies';
-import {post} from '../api/HTTP';
+import {gbkFetch} from '../api/HTTP';
 
 export default class YooLogin extends Component {
   constructor(props) {
@@ -24,7 +24,7 @@ export default class YooLogin extends Component {
   }
 
   login(onFail) {
-    post('http://eol.ctbu.edu.cn/meol/loginCheck.do', {
+    gbkFetch('POST', 'http://eol.ctbu.edu.cn/meol/loginCheck.do', {
       headers: {
         'Cache-Control': 'max-age=0',
         'Upgrade-Insecure-Requests': '1',

src/component/YooSplash.js

@@ -1,12 +1,12 @@
 import React, {Component} from 'react';
 import {ToastAndroid} from 'react-native';
 import YooSplashUI from '../ui/YooSplashUI';
-import {post} from '../api/HTTP';
+import {gbkFetch} from '../api/HTTP';
 import CookieManager from '@react-native-community/cookies';
 
 export default class YooSplash extends Component {
   componentDidMount() {
-    post('http://eol.ctbu.edu.cn/meol/loginCheck.do', {
+    gbkFetch('POST', 'http://eol.ctbu.edu.cn/meol/loginCheck.do', {
       headers: {
         'Cache-Control': 'max-age=0',
         'Upgrade-Insecure-Requests': '1',