Set Expires Header only if upstream has not already set an Expires

[Thomas Glanzmann]

Hello,
I would like to add an Expires Header only to upstream content that has
not already set an Expires header. Is there an easy way to do that with
nginx?

I thought about trying to add a header_filter_by_lua checking the
Expires header and set the necessary value if not already set. Is there
an easier way to do the same?

Cheers,
        Thomas