Bug 22343: Make C4::Letters use the new SMTP server config

Bug 22343: Make C4::Letters use the new SMTP server config

This patch makes the different methods in C4::Letters use:
- Koha::SMTP::Servers: to get the effective SMTP server for the library
  or the fallback default if no library in context.
- New Koha::Email->create method for crafting the email envelope for
  sending.

The tests are adapted so they behave the same way, but the trapped (in
the mock) $email object has the right type and its attributes are
accessed correctly.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Letters.t
=> SUCCESS: Tests pass. YAY!
3. Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>