load_on_pending is different from enable_relationship_loading and should
not be superseded.  both have a potential use.
- add "force_nocheck" as a way to turn on unicode=force without even
doing the isinstance() check - currently used only by psycopg2 + native enum + py2k.
- didn't realize psycopg2 had UNICODEARRAY extension all this time; replace _PGArray
with just using UNICODEARRAY instead.
- replace unnecessary/inconsistent __import__ in _isolation_lookup.
wrong method name
- Fixed bug when using joined table inheritance from a table to a
select/alias on the base, where the PK columns were also not same
named; the persistence system would fail to copy primary key values
from the base table to the inherited table upon INSERT.
[ticket:2885]