Bug 29420: HTTP status code incorrect when calling error pages directly under Plack...

Bug 29420: HTTP status code incorrect when calling error pages directly under Plack/PSGI

The error pages wrote a HTTP status code of 200 for all PSGI requests, even
though it should have only done it for PSGI requests from the ErrorDocument
middleware. This patch fixes that.

0) Do not apply patch
1) Open F12 dev tools and go to Network tab
2) Go to http://localhost:8081/files/blah
3) Note that the webpage is a 404 error but HTTP status code is 200
4) Go to http://localhost:8081/cgi-bin/koha/circ/blah
5) Note that the webpage is a 404 error and HTTP status code is 404

6) Apply patch
7) Go to http://localhost:8081/files/blah
8) Note that the webpage is a 404 error and HTTP status code is 404
9) Go to http://localhost:8081/cgi-bin/koha/circ/blah
10) Note that the webpage is a 404 error and HTTP status code is 404

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>