Bug 32995: Pass user agent string for OverDrive fulfillment

Bug 32995: Pass user agent string for OverDrive fulfillment

This patch simply adds the string Koha/VERSION to the fulfillment
request

TO test:
1 - Have OverDrive credentials for testing
2 - Enable all the overdrive options in system preferences
3 - Sign in to opac with an overdrive user and verify you can checkout
  and download titles
4 - Sign off

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Not tested with OverDrive, but change looks harmless enough.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>