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
Perhaps we should have used `ngx_conf_log_error`; I think that `cycle->log` isn't appropriate to use at this point.