Fixed some deprecation messages

[stollr]

Fixed the following two deprecation messages, which are reported by Symfony's deprecation helper:

1x: Method "Pimple\Container::offsetGet()" might add "mixed" as a native return type declaration in the future. Do the same in child class "ZBateson\MailMimeParser\Container" now to avoid errors or add an explicit @return annotation to suppress this message.

1x: Method "ArrayAccess::offsetGet()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "ZBateson\MailMimeParser\Message\PartChildrenContainer" now to avoid errors or add an explicit @return annotation to suppress this message.

The return annotations were already added by #197, but reverted again with commit c1c4b68 (PHPCSFixer).

[zbateson]

thank you, released in 2.4.1.