Changes with nginx 1.13.9 20 Feb 2018
*) Feature: HTTP/2 server push support; the "http2_push" and
"http2_push_preload" directives.
*) Bugfix: "header already sent" alerts might appear in logs when using
cache; the bug had appeared in 1.9.13.
*) Bugfix: a segmentation fault might occur in a worker process if the
"ssl_verify_client" directive was used and no SSL certificate was
specified in a virtual server.
*) Bugfix: in the ngx_http_v2_module.
*) Bugfix: in the ngx_http_dav_module.
