public function testAppendHeaderAddsToExistingHeaderIfAlreadyExists()
{
$msg = $this->getMockForAbstractClass('Amp\\Artax\\Message');
$msg->appendHeader('My-Header', 'val1');
$this->assertEquals(['val1'], $msg->getHeader('my-header'));
$msg->appendHeader('my-heAder', ['val2']);
$msg->appendHeader('MY-HEADER', 'val3');
$this->assertEquals(['val1', 'val2', 'val3'], $msg->getHeader('my-header'));
}