fix v-if contained check inside transcluded content (fix #1291)

Author: yyx990803

src/directives/if.js

@@ -82,7 +82,7 @@ module.exports = {
   },
 
   getContainedComponents: function () {
-    var vm = this.vm
+    var vm = this._host || this.vm
     var start = this.start.nextSibling
     var end = this.end