Bug 27424: (QA follow-up) Make Koha::SMTP::Server->store deal with defaults

Bug 27424: (QA follow-up) Make Koha::SMTP::Server->store deal with defaults

This patch makes the ->store method take care of setting the server as
the only default one.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/SMTP/Server.t
=> SUCCESS: Tests pass!
3. The UI keeps working as it should.
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>