Bug 16520: Allow per-VirtualHost environment variables with Plack

Bug 16520: Allow per-VirtualHost environment variables with Plack

This allows OVERRIDE_SYSPREF_* and others to work properly.

Test plan:

  1) Add the following line to your plack.psgi (near the bottom, just
     above "mount ..."):
       enable "+Koha::Middleware::Plack";
  2) Load the OPAC advanced search page (under Plack). The title should
    read "Koha online catalog" (or whatever your LibraryName syspref
    contains).
  3) Add the following to your Apache configuration:
       RequestHeader add X-Koha-SetEnv "OVERRIDE_SYSPREF_LibraryName Potato\, Potato"
  4) Restart Apache.
  5) Refresh. The title should now read "Potato, Potato online catalog".

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>