Allow rendering forbidden tags on the server (#3425)

src/compiler/parser/index.js

@@ -85,7 +85,7 @@ export function parse (
         element.ns = ns
       }
 
-      if (isForbiddenTag(element)) {
+      if (process.env.VUE_ENV !== 'server' && isForbiddenTag(element)) {
         element.forbidden = true
         process.env.NODE_ENV !== 'production' && warn(
           'Templates should only be responsbile for mapping the state to the ' +