fix ssr style registration

Author: yyx990803

lib/addStylesServer.js

@@ -5,21 +5,16 @@ module.exports = function (parentId, list, isProduction, context) {
     context = __VUE_SSR_CONTEXT__
   }
   if (context) {
-    var styles = context._styles
-
-    if (!styles) {
-      styles = context._styles = {}
+    if (!context.hasOwnProperty('styles')) {
       Object.defineProperty(context, 'styles', {
         enumberable: true,
-        get : function() {
-          return (
-            context._renderedStyles ||
-            (context._renderedStyles = renderStyles(styles))
-          )
+        get: function() {
+          return renderStyles(context._styles)
         }
       })
     }
 
+    var styles = context._styles || (context._styles = {})
     list = listToStyles(parentId, list)
     if (isProduction) {
       addStyleProd(styles, list)