fix(core): merge user defined class with component top level class (#403)

Author: farnabaz

src/core/runtime/components/NuxtContent.vue

@@ -3,7 +3,7 @@ import { pascalCase } from 'scule'
 import Vue from 'vue'
 import info from 'property-information'
 
-const rootKeys = ['class-name', 'class', 'style']
+const rootKeys = ['class-name', 'class', 'className', 'style']
 
 const rxOn = /^@|^v-on:/
 const rxBind = /^:|^v-bind:/