Bug 16615: Open Library API fails for full-site-SSL

Bug 16615: Open Library API fails for full-site-SSL

When running Koha in a full-site-SSL configuration accessing
external resouces like Open Library cover images via HTTP
doesn't work (mixed SSL/non-SSL content). It thus breaks a
core feature of Koha. Referencing such resources always via
HTTPS resolves the problem and also enhances privacy as a
side-effect.

Previous tickets took care of Amazon resources. This ticket
is supposed to address the remaining external resource providers,
starting with Open Library.

TEST PLAN
---------
1) Over https access your OPAC with the Enhanced Content system
   preferences 'OpenLibraryCovers' set to 'Add', and
   'OpenLibrarySearch' set to 'Show'.
2) Search for something (e.g. Mo Willems)
   -- Your browser console log will show messages about
      mixed content.
3) Apply the patch
4) Making sure to reload the java script, attempt steps 1 & 2
   again.
   -- no mixed content messages should be given
5) run koha qa test tools

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>