mod_lua: Fix r:setcookie() to add, rather than replace,

the Set-Cookie header. PR56105

Submitted By: Kevin J Walters <kjw ms com>, Edward Lu <Chaosed0 gmail com>
Committed By: covener



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1567221 13f79535-47bb-0310-9956-ffa450edef68