mod_session_cookie: avoid adding the Set-Cookie header

mod_session_cookie: avoid adding the Set-Cookie header
                    in both r->headers_out and r->err_headers_out
                    to avoid duplication.

In session_cookie_save it seems that ap_cookie_write is called
with r->headers_out and r->err_headers_out, ending up in the same
Set-Cookie header on both tables and eventually duplicated in the
HTTP response. I took Emmanuel's patch and trimmed out the bits
that remove the header only from r->err_headers_out (leaving it
to do the work on both tables) as attempt to change this bit of code
in the most conservative way as possible. Sending a commit for
a broader review.

PR: 60910,56098,55278

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1843244 13f79535-47bb-0310-9956-ffa450edef68