Thumbnails created with "{{thumbnail(image.png)}}" are always generated with absolute HTTP-path, even if the Redmine runs with HTTPS.

Solution: If thumbnail path had been relative, we wouldn't have this problem. (Like thumbnails from attachments.)

Environment:
  Redmine version                2.5.2.stable
  Ruby version                   2.1.3-p242 (2014-09-19) [x86_64-linux]
  Rails version                  3.2.19
  Environment                    production
  Database adapter               Mysql2