Add a deleteById test

test/model.test.js

@@ -192,7 +192,21 @@ describe('Model', function() {
     });
   });
 
-  describe('Model.destroyAll(callback)', function() {
+    describe('Model.deleteById([callback])', function () {
+        it("Delete a model instance from the attached data source", function (done) {
+            User.create({first: 'joe', last: 'bob'}, function (err, user) {
+                User.deleteById(user.id, function (err) {
+                    User.findById(user.id, function (err, notFound) {
+                        assert(!err);
+                        assert.equal(notFound, null);
+                        done();
+                    });
+                });
+            });
+        });
+    });
+
+    describe('Model.destroyAll(callback)', function() {
     it("Delete all Model instances from data source", function(done) {
       (new TaskEmitter())
         .task(User, 'create', {first: 'jill'})