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

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:
```php
$message = new PAMI\Message\Response\GenericResponse("Foo: 123\r\n");
$fooValue = $message->getKey('Foo');

echo gettype($fooValue); // => integer
```