Bug 16644: Plack: Use to_app to remove warning about Plack::App::CGIBin instance

Bug 16644: Plack: Use to_app to remove warning about Plack::App::CGIBin instance

Using Plack with the debian psgi file, I get lots of warnings like:
WARNING: Automatically converting Plack::App::CGIBin instance to a PSGI code reference. If you see this warning for each request, you probably need to explicitly call to_app() i.e. Plack::App::CGIBin->new(...)->to_app in your PSGI file.

This patch is aimed to eliminate the warns.

Test plan:
Run Plack with plack.psgi or koha.psgi and verify if you do not see these
warnings anymore.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
I tested on Jessie and I see no regressions.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>