public function testCanRemoveHeaders()
{
$message = $this->message->withHeader('X-Foo', 'Foo');
$this->assertNotSame($this->message, $message);
$this->assertTrue($message->hasHeader('x-foo'));
$message2 = $message->withoutHeader('x-foo');
$this->assertNotSame($this->message, $message2);
$this->assertNotSame($message, $message2);
$this->assertFalse($message2->hasHeader('X-Foo'));
}