fix(bind): check for where function when binding

Should correctly detect if the ref is a collection or a document

fix #146

Author: posva

src/index.js

@@ -65,7 +65,7 @@ function bindDocument ({
 
 function bind ({ vm, key, ref }) {
   let unbind
-  if (ref.add) {
+  if (ref.where) {
     unbind = bindCollection({
       vm,
       key,

test/helpers/mock.js

@@ -87,6 +87,9 @@ class CollectionReference {
     return this.data[id]
   }
 
+  // used to check if it's a collection or document ref
+  where () {}
+
   doc (id) {
     id = id || new Key()
     return this.data[id] = this.data[id] || new DocumentReference({