FIX: Reporting PDF encryption for non reference.

[carlwilson]

• check that encryption isn't a dictionary before reporting encryption messages;
• _encrypted set on the value of both reference OR dictionary not null;
• removed unused imports and added string constant for Specification title;
• added test files for non-indirect reference encryption:
  • test-root/corpora/errors/modules/PDF-hul/issue6010_1.pdf;
  • test-root/corpora/errors/modules/PDF-hul/issue6010_2.pdf; and
• added results copy for new encryption tests.