[5.4] Container: Add a new method to remove extenders of abstract

[LeoYang90]

public function testExtendAfterResolve()
{
    $container = new Container;
    $container->bind('foo', function () {
        $obj = new StdClass;
        $obj->foo = 'bar';

        return $obj;
    });

    $container->extend('foo', function ($obj, $container) {
        $obj->bar = 'baz';

        return $obj;
    });

    unset($container['foo']);

    $container->bind('foo', function () {
        return 'foo';
    });

    $obj = $container->make('foo');
}

It will throw a exception 'Attempt to assign property of non-object'.
If we want to rebind the abstract that has been changed by extend(), we can not clear the $extenders[]. So we can never rebind 'foo' to a closure that returns string likes 'foo'.

[ntzm]

This if statement is unnecessary, unset will do the check automatically