Bug 9452: C4::Letters not Plack-compatible

Bug 9452: C4::Letters not Plack-compatible

Prior to this patch, at more-or-less random intervals pages working
with notices will cease to function. To test:

1) Apply patch.
2) Try to edit some notices.
3) Trigger some notices.
4) If you were able to edit the notices and trigger the notices, sign
   off.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
All tests and QA script pass.
I did a regression script without Plack:
- edit, add, delete and copy notice
- trigger checkout/checkin notice
- print issueslip
No problems found.
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>