Fix #2296 get works with 0 as a model id

backbone.js

@@ -726,7 +726,7 @@
     // Get a model from the set by id.
     get: function(obj) {
       if (obj == null) return void 0;
-      return this._byId[obj.id || obj.cid || obj];
+      return this._byId[obj.id != null ? obj.id : obj.cid || obj];
     },
 
     // Get the model at the given index.

test/collection.js

@@ -62,8 +62,10 @@ $(document).ready(function() {
     strictEqual(collection.last().get('a'), 4);
   });
 
-  test("get", 5, function() {
+  test("get", 6, function() {
     equal(col.get(0), d);
+    var d2 = d.clone();
+    equal(col.get(d2), d);
     equal(col.get(2), b);
     equal(col.get({id: 1}), c);
     equal(col.get(c.clone()), c);