Merge pull request #2162 from elsassph/fix-2161-dev

Fixes: __vfrag__ references prevent GC of Vue objects

Author: yyx990803

src/fragment/fragment.js

@@ -181,6 +181,7 @@ Fragment.prototype.destroy = function () {
   if (this.parentFrag) {
     this.parentFrag.childFrags.$remove(this)
   }
+  this.node.__vfrag__ = null
   this.unlink()
 }