Return value of COSDocument#isEncrypted is unclear

According to the java doc COSDocument#isEncrypted provides if a pdf is encrypted or not. But it's not clear what it really means:

• is the pdf somehow encrypted
• is an encrypted pdf already decrypted after parsing

I guess the first case was the origin intention of this method. Although a comment in the code indicates that the information is needed within the non-sequential parser I can't find any call of COSDocument#isEncrypted anywhere in the non-sequential parser.

I'm going to change the behaviour of isEncrypted so that it follows the origin intention and I'll add a new method isDecrypted as well.