feat(http2): push assets with single link header

less payload!

https://blog.cloudflare.com/http-2-server-push-with-multiple-assets-per-link-header

lib/render.js

@@ -87,7 +87,9 @@ export async function render (req, res) {
           pushAssets.push(`<${href}>; rel=${rel}; as=${as}`)
         }
       }
-      res.setHeader('Link', pushAssets)
+      // Pass with single Link header
+      // https://blog.cloudflare.com/http-2-server-push-with-multiple-assets-per-link-header
+      res.setHeader('Link', pushAssets.join(','))
     }
     res.setHeader('Content-Type', 'text/html; charset=utf-8')
     res.setHeader('Content-Length', Buffer.byteLength(html))