This is fixed if a static file is being served. or when the content served by
serve_file() is an actual file on disk.

If the content served by serve_file() is not an actual file (e.g. content in a database
or an S3 bucket), only the Last-Modified header is emitted.

This change also supports reporting 304 with the if-none-match header if it has just
a single etag value without spaces around it. This could be made more forgiving
and support a list of etags by using the etag functions from the rest.py module.

However currently the etag is not content-encoding aware. So it doesn't have the -gzip
suffix used by the rest interface.

But the initial issue is solved and redbot is happy.