Encrypt /DA string when document is encrypted

[JedrzejczykRobert]

When document is encrypted then /DA string must be encrypted, without this Acrobat cannot allow fill form fields.

[CLAassistant]

All committers have signed the CLA.

[JedrzejczykRobert]

Steps to reproduce:

use code from examples\example_054.php

before "$pdf->Output('example_054.pdf', 'D');" insert

$pdf->setProtection(array(), 'test', 'test', 3, null);