Comment 8 for bug 101759

From r23559:

Ghosts now override `get_modification_datetime` to get their
modification date from the haunted object, allowing the
`Last-Modified` header to be set in a convenient way.

If you're setting the headers via model/get_modification_datetime, this should
be your fix. Previously, this would have returned None.

Can you test this and confirm that we can close the issue? Thanks!