Removing expire date kills status view

[dgoetz]

I found that if a host has an expire date set and this gets removed, the status gets not cleared and the to_date in app/models/host_status/expiration_status.rb running on nil fails causing the status view to show only N/A for every state and showing no detail statuses.
If you reset the status for example by delete the database entry, everything is fine again.
Another option would be to always show a state like "OK - No expire date set" in case of no date is set.