Remove dublicate className from head (#5026)
Remove dublicate className from head (#5026)

This PR:

* Removes dublicate `className` in: `<meta charSet="utf-8" class="next-head"/>`
* Refactores head reducer

Special thanks to @udanpe for reporting and @geoffRGWilliams for bringing me on the right tack! 🙌

Closes #4745, #4802 

zeit/next.js,azukaru/next.js,azukaru/next.js,BlancheXu/test,flybayer/next.js,JeromeFitz/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js,BlancheXu/test,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,BlancheXu/test