Turns out that subrequests aren't pushed through Nginx's header filters which means that a directive like add_header won't work in a shib_request (or auth_request etc).  Good to know!

This reverts commit f5bfd9ea.

This ensures that Nginx can still manipulate responses from
the Shibboleth authorizer.

Signed-off-by: Luca Bruno <luca.bruno@rocket-internet.de>