Fix for issue #60

Not sure why git also included all these other commits, only the last one is relevant. Tests:

    public function testChangedWithSameData() {
        $animal = new Animal(array('type' => 'horse'));
        $animal->save();
        $animal->load(array('type' => 'horse'));
        $this->assertEquals(array(), $animal->changes());
    }

    public function testChangedWithChangedData() {
        $animal = new Animal(array('type' => 'horse'));
        $animal->save();
        $animal->load(array('type' => 'frog'));
        $this->assertEquals(array('type'), $animal->changes());
    }