Fix/amp example (vercel#1755)

* Using developit/unfetch as the Fetch API polyfill

* Fixed amp example checking if the amp attribute is already present

* Fix lint

examples/with-amp/pages/_document.js

@@ -1,13 +1,16 @@
 import Document, { Head } from 'next/document'
 import { DOMProperty } from 'react-dom/lib/ReactInjection'
+import { properties as DOMProperties } from 'react-dom/lib/DOMProperty'
 
 // By default React limit the set of valid DOM elements and attributes
 // (https://github.com/facebook/react/issues/140) this config whitelist
 // Amp elements/attributes
-DOMProperty.injectDOMPropertyConfig({
-  Properties: { amp: DOMProperty.MUST_USE_ATTRIBUTE },
-  isCustomAttribute: attributeName => attributeName.startsWith('amp-')
-})
+if (typeof DOMProperties.amp === 'undefined') {
+  DOMProperty.injectDOMPropertyConfig({
+    Properties: { amp: DOMProperty.MUST_USE_ATTRIBUTE },
+    isCustomAttribute: attributeName => attributeName.startsWith('amp-')
+  })
+}
 
 export default class MyDocument extends Document {
   render () {