Dedupe only items with unique key (#5800)

Fixes #3705
Fixes #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.

packages/next-server/lib/head.js

@@ -52,8 +52,7 @@ function onStateChange (head) {
   }
 }
 
-const METATYPES = ['name', 'httpEquiv', 'charSet', 'itemProp', 'property']
-const ALLOWED_DUPLICATES = ['article:tag', 'og:image', 'og:image:alt', 'og:image:width', 'og:image:height', 'og:image:type', 'og:image:secure_url', 'og:image:url']
+const METATYPES = ['name', 'httpEquiv', 'charSet', 'itemProp']
 
 /*
  returns a function for filtering head child elements
@@ -71,6 +70,7 @@ function unique () {
     if (h.key && h.key.indexOf('.$') === 0) {
       if (keys.has(h.key)) return false
       keys.add(h.key)
+      return true
     }
     switch (h.type) {
       case 'title':
@@ -89,7 +89,7 @@ function unique () {
           } else {
             const category = h.props[metatype]
             const categories = metaCategories[metatype] || new Set()
-            if (categories.has(category) && ALLOWED_DUPLICATES.indexOf(category) === -1) return false
+            if (categories.has(category)) return false
             categories.add(category)
             metaCategories[metatype] = categories
           }

test/integration/basic/pages/head.js

@@ -13,8 +13,8 @@ export default () => <div>
     {/* allow duplicates for specific tags */}
     <meta property='article:tag' content='tag1' key='tag1key' />
     <meta property='article:tag' content='tag2' key='tag2key' />
-    <meta property='dedupe:tag' content='tag3' key='tag3key' />
-    <meta property='dedupe:tag' content='tag4' key='tag4key' />
+    <meta property='dedupe:tag' content='tag3' key='same-key' />
+    <meta property='dedupe:tag' content='tag4' key='same-key' />
     <meta property='og:image' content='ogImageTag1' key='ogImageTag1Key' />
     <meta property='og:image' content='ogImageTag2' key='ogImageTag2Key' />
     <meta property='og:image:alt' content='ogImageAltTag1' key='ogImageAltTag1Key' />
@@ -30,6 +30,9 @@ export default () => <div>
     <meta property='og:image:url' content='ogImageUrlTag1' key='ogImageUrlTag1Key' />
     <meta property='og:image:url' content='ogImageUrlTag2' key='ogImageUrlTag2Key' />
 
+    <meta property='fb:pages' content='fbpages1' />
+    <meta property='fb:pages' content='fbpages2' />
+
     <React.Fragment>
       <title>Fragment title</title>
       <meta content='meta fragment' />
@@ -43,5 +46,5 @@ export default () => <div>
     <link rel='stylesheet' href='dedupe-style.css' key='my-style' />
     <link rel='stylesheet' href='dedupe-style.css' key='my-style' />
   </Head>
-  <h1>I can haz meta tags</h1>
+  <h1>I can have meta tags</h1>
 </div>

test/integration/basic/test/rendering.js

@@ -33,7 +33,7 @@ export default function ({ app }, suiteName, render, fetch) {
       const html = await (render('/head'))
       expect(html.includes('<meta charSet="iso-8859-5" class="next-head"/>')).toBeTruthy()
       expect(html.includes('<meta content="my meta" class="next-head"/>')).toBeTruthy()
-      expect(html.includes('I can haz meta tags')).toBeTruthy()
+      expect(html.includes('I can have meta tags')).toBeTruthy()
     })
 
     test('header helper dedupes tags', async () => {
@@ -65,7 +65,8 @@ export default function ({ app }, suiteName, render, fetch) {
       expect(html).toContain('<meta property="og:image:secure_url" content="ogImageSecureUrlTag1" class="next-head"/>')
       expect(html).toContain('<meta property="og:image:secure_url" content="ogImageSecureUrlTag2" class="next-head"/>')
       expect(html).toContain('<meta property="og:image:url" content="ogImageUrlTag1" class="next-head"/>')
-      expect(html).toContain('<meta property="og:image:url" content="ogImageUrlTag2" class="next-head"/>')
+      expect(html).toContain('<meta property="fb:pages" content="fbpages1" class="next-head"/>')
+      expect(html).toContain('<meta property="fb:pages" content="fbpages2" class="next-head"/>')
     })
 
     test('header helper renders Fragment children', async () => {