Fix/encrypt numbers

PR https://github.com/zendframework/zf2/pull/5436/files#diff-acc52fa0c89428ade03c42a52bd93c19 introduced a ~~BC break~~ bug to the Filter\Encrypt ~~and Filter\Decrypt~~. Before was allowed to encrypt/~~decrypt~~ ~~integers~~ numbers, after that PR it's restricted to strings only.

https://github.com/zendframework/zf2/blob/4b496e98fcb2a40165a738459a17fb885474d717/library/Zend/Filter/Encrypt.php#L126
~~https://github.com/zendframework/zf2/blob/4b496e98fcb2a40165a738459a17fb885474d717/library/Zend/Filter/Decrypt.php#L27~~

And I can't mention that silently returning a value when that condition isn't met is nasty, debugging that was annoying.