- state.commit() and state.commit_all() now reconcile the current dict against expire...

- state.commit() and state.commit_all() now reconcile the current dict against expired_attributes
and unset the expired flag for those attributes.  This is partially so that attributes are not
needlessly marked as expired after a two-phase inheritance load.
- fixed bug which was introduced in 0.4.3, whereby loading an
already-persistent instance mapped with joined table inheritance
would trigger a useless "secondary" load from its joined
table, when using the default "select" polymorphic_fetch.
This was due to attributes being marked as expired
during its first load and not getting unmarked from the
previous "secondary" load.  Attributes are now unexpired
based on presence in __dict__ after any load or commit
operation succeeds.