avoid having both 'keep' and 'expires_at' (#484)

Remove 'expires_at' if there is also 'keep', since 'keep' currently
takes precendence.

We'll want to remove the 'keep' entries for most of these cases eventually,
but let's do that gradually to reduce impact.

See also #252

Signed-off-by: Arnout Engelen <engelen@apache.org>

Author: raboof