public function testIsEmpty()
 {
     $container = new Configuration();
     $this->assertTrue($container->isEmpty());
     $container->foo = 'bar';
     $this->assertFalse($container->isEmpty());
     unset($container->foo);
     $this->assertTrue($container->isEmpty());
     $container['foo'] = 'bar';
     $this->assertFalse($container->isEmpty());
     unset($container['foo']);
     $this->assertTrue($container->isEmpty());
 }