This PR fixes the buggy `Head.propTypes` here:
https://github.com/zeit/next.js/blob/v8.0.0-canary.3/packages/next-server/lib/head.js#L107
Currently, `Head.propTypes` allows one child node like this:
```jsx
import Head from 'next/head'
// …
<Head>
<title>Title</title>
</Head>
```
But more than one child node mistakenly causes a prop type error like this:
```jsx
<Head>
<title>Title</title>
<meta name="description" content="Description." />
</Head>
```
```
Warning: Failed prop type: Invalid prop `children` supplied to `Head`.
```
Fixes#3705Fixes#4656
- No longer automatically dedupe certain tags. Only the ones we know are *never* going to be duplicate like charSet, title etc.
- Fix `key=""` behavior, making sure that if a unique key is provided tags are deduped based on that.
For example:
```jsx
<meta property='fb:pages' content='one'>
<meta property='fb:pages' content='two'>
```
Would currently cause
```jsx
<meta property='fb:pages' content='two'>
```
### After this change:
```jsx
<meta property='fb:pages' content='one'>
<meta property='fb:pages' content='two'>
```
Then if you use next/head multiple times / want to be able to override:
```jsx
<meta property='fb:pages' content='one' key="not-unique-key">
<meta property='fb:pages' content='two' key="not-unique-key">
```
Would cause:
```jsx
<meta property='fb:pages' content='two'>
```
As `key` gets deduped correctly after this PR, similar to how React itself works.
