关于Nginx高性能Web服务器详解一书的一处错误

详情：此书第52页中提到ngx_http_gzip_module为标准HTTP模块。而在第53页中此模块变成了可选HTTP模块，自相矛盾，查了官网，此模块确实为标准HTTP模块，即会默认安装。

The ngx_http_gzip_module module is a filter that compresses responses using the “gzip” method. This often helps to reduce the size of transmitted data by half or even more.

http://nginx.org/en/docs/http/ngx_http_gzip_module.html#gzip