Problem: hydra_post_dup did not check null properties
Problem: hydra_post_dup did not check null properties

Solution: don't duplicate properties in a post that haven't been
set yet (and are null). Specifically, location and mime type.

Fixes #55

jemc/hydra,ZyreApps/hydra,taotetek/hydra,febeling/hydra,febeling/hydra,febeling/hydra,jemc/hydra,edgenet/hydra,ZyreApps/hydra,taotetek/hydra,ZyreApps/hydra,edgenet/hydra,febeling/hydra,jemc/hydra,jemc/hydra,edgenet/hydra,jemc/hydra,edgenet/hydra,taotetek/hydra,taotetek/hydra,febeling/hydra,ZyreApps/hydra,ZyreApps/hydra,taotetek/hydra,edgenet/hydra