Corrected the use of ap_table_set and ap_table_merge:

- 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.
- Fix a problem with proxy where X-Cache headers were
overwriting and then obliterating upstream X-Cache headers
from other proxies.
PR:
Obtained from:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@93364 13f79535-47bb-0310-9956-ffa450edef68