Fix regression from 1.3:

Fix regression from 1.3:

There's a new optional flag for the Header directive
('always'), which keeps the former ErrorHeader functionality.

The opposite flag to 'always' is 'onsuccess'. So the new directives
are written as:

Header set foo bar
# which is the same as
Header onsuccess set foo bar
# and a subset of
Header always set foo bar
# which is what the ErrorHeader directive did.

PR: 28657
Obtained from: 2.1
Reviewed by: Brad Nicholes, Joe Orton

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@104732 13f79535-47bb-0310-9956-ffa450edef68