Bug 18741: Fix loading of SQL files in web installer

Bug 18741: Fix loading of SQL files in web installer

Test plan:
1. Start a normal install using the web installer
2. When asked to select data to load, verify that
   'message_transport_types' is checked, and check some optional data
   files
3. Continue the install to the end
4. Verify that message_transport_types is not empty, and that the
   optional data you checked is there

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>