Combining unknown headers during variables lookup (ticket #1316). Previously, $http_*, $sent_http_*, $sent_trailer_*, $upstream_http_*, and $upstream_trailer_* variables returned only the first header (with a few specially handled exceptions: $http_cookie, $http_x_forwarded_for, $sent_http_cache_control, $sent_http_link). With this change, all headers are returned, combined together. For example, $http_foo variable will be "a, b" if there are "Foo: a" and "Foo: b" headers in the request. Note that $upstream_http_set_cookie will also return all "Set-Cookie" headers (ticket #1843), though this might not be what one want, since the "Set-Cookie" header does not follow the list syntax (see RFC 7230, section 3.2.2).