next/head allow duplicates if it has uniq keys (#4121)

resolves #4115

For now, I just added `'article:tag'` so it could be duplicated if we need more we have to extend:
```javascript
const ALLOWED_DUPLICATES = ['article:tag']
```

lib/head.js

@@ -49,9 +49,13 @@ function onStateChange (head) {
 }
 
 const METATYPES = ['name', 'httpEquiv', 'charSet', 'itemProp', 'property']
+const ALLOWED_DUPLICATES = ['article:tag']
 
-// returns a function for filtering head child elements
-// which shouldn't be duplicated, like <title/>.
+/*
+ returns a function for filtering head child elements
+ which shouldn't be duplicated, like <title/>,
+ except we explicit allow it in ALLOWED_DUPLICATES array
+*/
 
 function unique () {
   const keys = new Set()
@@ -81,7 +85,7 @@ function unique () {
           } else {
             const category = h.props[metatype]
             const categories = metaCategories[metatype] || new Set()
-            if (categories.has(category)) return false
+            if (categories.has(category) && ALLOWED_DUPLICATES.indexOf(category) === -1) return false
             categories.add(category)
             metaCategories[metatype] = categories
           }

test/integration/basic/pages/head.js

@@ -10,6 +10,12 @@ export default () => <div>
 
     <meta content='my meta' />
 
+    {/* 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' />
+
     <React.Fragment>
       <title>Fragment title</title>
       <meta content='meta fragment' />

test/integration/basic/test/rendering.js

@@ -43,6 +43,14 @@ export default function ({ app }, suiteName, render, fetch) {
       expect(html).not.toContain('<link rel="stylesheet" href="dedupe-style.css" class="next-head"/><link rel="stylesheet" href="dedupe-style.css" class="next-head"/>')
     })
 
+    test('header helper avoids dedupe of specific tags', async () => {
+      const html = await (render('/head'))
+      expect(html).toContain('<meta property="article:tag" content="tag1" class="next-head"/>')
+      expect(html).toContain('<meta property="article:tag" content="tag2" class="next-head"/>')
+      expect(html).not.toContain('<meta property="dedupe:tag" content="tag3" class="next-head"/>')
+      expect(html).toContain('<meta property="dedupe:tag" content="tag4" class="next-head"/>')
+    })
+
     test('header helper renders Fragment children', async () => {
       const html = await (render('/head'))
       expect(html).toContain('<title class="next-head">Fragment title</title>')