docs: no longer recommend polyfill.io

Author: SukkaW

README.md

@@ -83,7 +83,7 @@ Browser-ready versions of this module are available via [jsdelivr][], [unpkg][],
 This is the solution for you if you're just using `<script>` tags everywhere!
 
 ```html
-<script src="https://polyfill.io/v3/polyfill.min.js?features=WeakRef,BigInt"></script>
+<script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=WeakRef,BigInt"></script>
 <script src="https://cdn.jsdelivr.net/npm/superagent"></script>
 <!-- if you wish to use unpkg.com instead: -->
 <!-- <script src="https://unpkg.com/superagent"></script> -->
@@ -155,10 +155,10 @@ If you are using [browserify][], [webpack][], [rollup][], or another bundler, th
 
 ### Required Browser Features
 
-We recommend using <https://polyfill.io> (specifically with the bundle mentioned in [VanillaJS](#vanillajs) above):
+We recommend using <https://cdnjs.cloudflare.com/polyfill/> (specifically with the bundle mentioned in [VanillaJS](#vanillajs) above):
 
 ```html
-<script src="https://polyfill.io/v3/polyfill.min.js?features=WeakRef,BigInt"></script>
+<script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=WeakRef,BigInt"></script>
 ```
 
 * WeakRef is not supported in Opera 85, iOS Safari 12.2-12.5

docs/index.md

@@ -868,10 +868,10 @@ Libraries like [co](https://github.com/tj/co) or a web framework like [koa](http
 
 Note that SuperAgent expects the global `Promise` object to be present. You'll need to use v7 and a polyfill to use promises in Internet Explorer or Node.js 0.10.
 
-We have dropped support in v8 for IE.  You must add a polyfill for WeakRef and BigInt if you wish to support Opera 85, iOS Safari 12.2-12.5, for example using <https://polyfill.io>:
+We have dropped support in v8 for IE.  You must add a polyfill for WeakRef and BigInt if you wish to support Opera 85, iOS Safari 12.2-12.5, for example using <https://cdnjs.cloudflare.com/polyfill/>:
 
 ```html
-<script src="https://polyfill.io/v3/polyfill.min.js?features=WeakRef,BigInt"></script>
+<script src="https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=WeakRef,BigInt"></script>
 ```
 
 ## Browser and node versions

index.html

@@ -623,8 +623,8 @@ <h2 id="promise-and-generator-support">Promise and Generator support</h2>
     const res = yield req;
 </code></pre>
 <p>Note that SuperAgent expects the global <code>Promise</code> object to be present. You&#39;ll need to use v7 and a polyfill to use promises in Internet Explorer or Node.js 0.10.</p>
-<p>We have dropped support in v8 for IE.  You must add a polyfill for WeakRef and BigInt if you wish to support Opera 85, iOS Safari 12.2-12.5, for example using <a href="https://polyfill.io">https://polyfill.io</a>:</p>
-<pre><code class="language-html">&lt;script src=&quot;https://polyfill.io/v3/polyfill.min.js?features=WeakRef,BigInt&quot;&gt;&lt;/script&gt;
+<p>We have dropped support in v8 for IE.  You must add a polyfill for WeakRef and BigInt if you wish to support Opera 85, iOS Safari 12.2-12.5, for example using <a href="https://cdnjs.cloudflare.com/polyfill/">https://cdnjs.cloudflare.com/polyfill/</a>:</p>
+<pre><code class="language-html">&lt;script src=&quot;https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=WeakRef,BigInt&quot;&gt;&lt;/script&gt;
 </code></pre>
 <h2 id="browser-and-node-versions">Browser and node versions</h2>
 <p>SuperAgent has two implementations: one for web browsers (using XHR) and one for Node.JS (using core http module). By default Browserify and WebPack will pick the browser version.</p>