Nginx 主线版 1.13.7 发布，修复了部分 Bug

Nginx 1.13.7 已发布，该版本是 mainline 版，即通常说的开发版。本次更新主要是修复了 bug，详细如下：

Bugfix: in the $upstream_status variable.
Bugfix: a segmentation fault might occur in a worker process if a backend returned a “101 Switching Protocols” response to a subrequest.
Bugfix: a segmentation fault occurred in a master process if a shared memory zone size was changed during a reconfiguration and the reconfiguration failed.
Bugfix: in the ngx_http_fastcgi_module.
Bugfix: nginx returned the 500 error if parameters without variables were specified in the “xslt_stylesheet” directive.
Workaround: “gzip filter failed to use preallocated memory” alerts appeared in logs when using a zlib library variant from Intel.
Bugfix: the “worker_shutdown_timeout” directive did not work when using mail proxy and when proxying WebSocket connections.