Can we can test like this?
```
http {
   # Don't set up gzip; with NgxGZipSetter this will happen automatically.
   server {
      # Test that gzip is enabled on this server.
   }
   server {
      gzip ...;
      gzip_types image/png;
      # Test that gzip is enabled on this server too, with the explicit configuration.
      # We would normally never gzip a png, so by seeing a gzipped png we know
      # the gzip_types is being respected.
   }
   server {
      pagespeed gzip ...;
      # Test that we can also handle this nested configuration.
   }
}