Fix/amp example (#1755)
Fix/amp example (#1755)

* Using developit/unfetch as the Fetch API polyfill

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

* Fix lint

zeit/next.js,flybayer/next.js,azukaru/next.js,callumlocke/next.js,kevmannn/next.js,flybayer/next.js,BlancheXu/test,nelak/next.js,flybayer/next.js,flybayer/next.js,arunoda/next.js,azukaru/next.js,BlancheXu/test,kpdecker/next.js,azukaru/next.js,zeit/next.js,BlancheXu/test,zeit/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,giacomorebonato/next.js,JeromeFitz/next.js