*) Change in internal API: now header lines are represented as linked
   lists.

*) Change: now nginx combines arbitrary header lines with identical
   names when sending to FastCGI, SCGI, and uwsgi backends, in the
   $r->header_in() method of the ngx_http_perl_module, and during lookup
   of the "$http_...", "$sent_http_...", "$sent_trailer_...",
   "$upstream_http_...", and "$upstream_trailer_..." variables.

*) Bugfix: if there were multiple "Vary" header lines in the backend
   response, nginx only used the last of them when caching.

*) Bugfix: if there were multiple "WWW-Authenticate" header lines in the
   backend response and errors with code 401 were intercepted or the
   "auth_request" directive was used, nginx only sent the first of the
   header lines to the client.

*) Change: the logging level of the "application data after close
   notify" SSL errors has been lowered from "crit" to "info".

*) Bugfix: connections might hang if nginx was built on Linux 2.6.17 or
   newer, but was used on systems without EPOLLRDHUP support, notably
   with epoll emulation layers; the bug had appeared in 1.17.5.
   Thanks to Marcus Ball.

*) Bugfix: nginx did not cache the response if the "Expires" response
   header line disabled caching, but following "Cache-Control" header
   line enabled caching.