gzip: enabling gzip on "pagespeed on"

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 an explicit configuration is detected the gzip configuration
set by pagespeed on is rolled back

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/238
I think that's a bug, but let's not worry about that for now.  Let's make gzip go on when pagespeed goes on, and if pagespeed is going on when it shouldn't we can fix gzip when we fix that.