isUniqueHeader example

switch ($this->headerRules[$name] ?? self::HEADER_SET_IF_EMPTY) {
                case self::HEADER_SET_IF_EMPTY:
                    break;

                case self::HEADER_REPLACE:
                    $headers->remove($name);
                    $headers->add($header);

                    break;

                case self::HEADER_ADD:
                    if (!Headers::isUniqueHeader($name)) {
                        $headers->add($header);

                        break;
                    }

                    $h = $headers->get($name);
                    if (!$h instanceof MailboxListHeader) {
                        throw new RuntimeException(sprintf('Unable to set header "%s".', $name));
                    }

                    Headers::checkHeaderClass($header);
                    

public function testAddHeaderIsNotCaseSensitive()
    {
        $headers = new Headers();
        $headers->addHeader('From', ['from@example.com']);

        $this->assertInstanceOf(MailboxListHeader::class, $headers->get('from'));
        $this->assertEquals([new Address('from@example.com')], $headers->get('from')->getBody());
    }

    public function testIsUniqueHeaderIsNotCaseSensitive()
    {
        $this->assertTrue(Headers::isUniqueHeader('From'));
    }

    public function testToStringJoinsHeadersTogether()
    {
        $headers = new Headers();
        $headers->addTextHeader('Foo', 'bar');
        $headers->addTextHeader('Zip', 'buttons');
        $this->assertEquals("Foo: bar\r\nZip: buttons\r\n", $headers->toString());
    }

    public function testHeadersWithoutBodiesAreNotDisplayed()
    {