PAMI\Message\Message::getKey() wrong return type in PHPDoc

[7-zete-7]

In PHPDoc return type is string, but PAMI\Message\Message::$keys can also contains any of types returned by PAMI\Message\Message::sanitizeInput() method.

Can be reproduced by code:

$message = new PAMI\Message\Response\GenericResponse("Foo: 123\r\n");
$fooValue = $message->getKey('Foo');

echo gettype($fooValue); // => integer