D9580: Fix saving annotations to gzipped files

[Albert Astals Cid]

aacid accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R223 Okular

BRANCH
  Applications/17.12

REVISION DETAIL
  https://phabricator.kde.org/D9580

To: rkflx, aacid, #okular
Cc: #okular, michaelweghorn, ngraham, aacid