function testAddHeaders()
 {
     $message = new MessageMock();
     $message->addHeaders(['a' => 'b']);
     $message->addHeaders(['c' => 'd']);
     $this->assertEquals('b', $message->getHeader('a'));
 }
 function testMultipleHeaders()
 {
     $message = new MessageMock();
     $message->setHeader('a', '1');
     $message->addHeader('A', '2');
     $this->assertEquals("1,2", $message->getHeader('A'));
     $this->assertEquals("1,2", $message->getHeader('a'));
     $this->assertEquals(['1', '2'], $message->getHeaderAsArray('a'));
     $this->assertEquals(['1', '2'], $message->getHeaderAsArray('A'));
     $this->assertEquals([], $message->getHeaderAsArray('B'));
 }