this adds the following configuration on "pagespeed on"

gzip on;
gzip_proxied any;
gzip_vary on;
gzip_types application/ecmascript;
gzip_types application/javascript;
gzip_types application/json;
gzip_types application/pdf;
gzip_types application/postscript;
gzip_types application/x-javascript;
gzip_types image/svg+xml;
gzip_types text/css;
gzip_types text/csv;
gzip_types text/html;
gzip_types text/javascript;
gzip_types text/plain;
gzip_types text/xml;
gzip_http_version 1.0;

If any explicit gzip configuration is detected the gzip configuration
set by pagespeed is rolled back completely and the explicit gzip
configuration is used.

To enable/disable gzip with pagespeed the following commands can be used:

pagespeed gzip on;
pagespeed gzip off;

We test the nesting of the gzip configuration (pagespeed gzip on/off and
pagespeed on/off)
Ideally we need to test all the content types as well and the rollback in
case of an explicit gzip configuration, but to do this we need to be able
to map a custom directory into the nginx tests this and seperate nginx
config files.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/238