base_asset: fixed AsOf member

The historic date a given object refers to was added to an object using
with_data(), however as it is missing from the meta information returned
by V1, it is not registered as an attribute, thus unavailable through
the object.

The solution is to use a normal instance variable, which can be set to
None when we are not querying the history.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
query: make it possible to query the actual state by passing None to asof()

Earlier if asof() was specified, it was impossible to list an entry to
represent the current state. This patch makes it possible to use
None to represent "NOW".

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>