Some browsers ignore cookies that have been merged into a

Some browsers ignore cookies that have been merged into a
single Set-Cookie header. Set-Cookie and Set-Cookie2 headers
are now unmerged in the http proxy before being sent to the
client.
Fix a problem with proxy where each entry of a duplicated
header such as Set-Cookie would overwrite and obliterate the
previous value of the header, resulting in multiple header
values (like cookies) going missing.
PR:
Obtained from:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93527 13f79535-47bb-0310-9956-ffa450edef68