Bug 33967: Fix SetEnv middleware to preserve $env integrity

Bug 33967: Fix SetEnv middleware to preserve $env integrity

This patch fixes the Koha::Middleware::SetEnv, so that it
preserves the referential integrity of the $env hash
reference as it passes through the pre-process and post-process
stages of middleware wrapped around the core Koha Plack application.

Test plan:
0a. Apply the patch
0b. vi /etc/koha/sites/kohadev/koha-conf.xml
0c. Uncomment "koha_trusted_proxies"
0d. Add "172.0.0.0/8" to the end of the "koha_trusted_proxies"
0e. koha-plack --restart kohadev
1. vi /etc/apache2/sites-enabled/kohadev.conf
2. Add the following to the OPAC virtualhost:
SetEnv OVERRIDE_SYSPREF_LibraryName "The Best, Truly the Best, Koha Library"
RequestHeader add X-Koha-SetEnv "OVERRIDE_SYSPREF_LibraryName The Best\, Truly the Best\, Koha Library"
RequestHeader add X-Forwarded-For "192.168.100.100"
3. service apache2 restart
4. tail -f /var/log/koha/kohadev/plack.log
5. Go to http://localhost:8080/
6. Note that the plack.log lists 192.168.100.100 as the client IP address
7. Note that the title of the OPAC webpage is
"The Best, Truly the Best, Koha Library Library catalog"

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>