mod_headers: Add 'Header SetIfEmpty header-name val' to only set

a header if it's not already there (can't do this with Edit)

trunk patch: http://svn.apache.org/1496338

Submitted by: covener
Reviewed by: jim, humbedooh


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1523270 13f79535-47bb-0310-9956-ffa450edef68