delete reactive property, in case it existed

Author: Leszek

Diff for: src/vuefire.js

@@ -110,6 +110,7 @@ function bind (vm, key, source) {
  */
 function bindAsArray (vm, key, source, cancelCallback) {
   var array = []
+  Vue.delete(vm, key)
   Vue.util.defineReactive(vm, key, array)
 
   var onAdd = source.on('child_added', function (snapshot, prevKey) {