fix($core): preserve original meta charset and viewport (vuejs#2332)

Author: sakihet

packages/@vuepress/core/lib/client/root-mixins/updateMeta.js

@@ -35,10 +35,26 @@ export default {
 
     getMergedMetaTags () {
       const pageMeta = this.$page.frontmatter.meta || []
+      const templateMeta = []
+      if (typeof document !== 'undefined') {
+        const templateMetaTags = document.querySelectorAll('meta')
+        templateMetaTags.forEach(tag => {
+          const names = tag.getAttributeNames()
+          if (names.length === 1) {
+            const attr = tag.getAttributeNames()[0]
+            const obj = {}
+            obj[attr] = tag.getAttribute(attr)
+            templateMeta.push(obj)
+          } else if (names.every(e => ['name', 'content'].includes(e))) {
+            templateMeta.push({ name: tag.name, content: tag.content })
+          } else if (names.every(e => ['http-equiv', 'content'].includes(e))) {
+            templateMeta.push({ 'http-equiv': tag.httpEquiv, content: tag.content })
+          }
+        })
+      }
       // pageMetaTags have higher priority than siteMetaTags
       // description needs special attention as it has too many entries
-      return unionBy([{ name: 'description', content: this.$description }],
-        pageMeta, this.siteMeta, metaIdentifier)
+      return unionBy([{ name: 'description', content: this.$description }], pageMeta, this.siteMeta, templateMeta, metaIdentifier)
     }
   },