Write 'Persistent-Auth' header to err_headers_out

In some cases, like internal redirects, authentication is completed but our
'Persistent-Auth' header is dropped by the server because headers_out is ignored
with errors (4xx, 5xx) and internal redirects.

See: https://ci.apache.org/projects/httpd/trunk/doxygen/structrequest__rec.html#a9f49c2d5680987c0c28466ea37d41a62

This fixes #110

Reviewed-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Isaac Boukris <iboukris@gmail.com>
Closes #111

Author: michael-o

src/mod_auth_gssapi.c

@@ -1049,7 +1049,7 @@ static int mag_auth(request_rec *req)
     ret = mag_complete(req_cfg, mc, client, mech_type, vtime, delegated_cred);
 
     if (ret == OK && req_cfg->send_persist)
-        apr_table_set(req->headers_out, "Persistent-Auth",
+        apr_table_set(req->err_headers_out, "Persistent-Auth",
             cfg->gss_conn_ctx ? "true" : "false");
 
 done: