- Modified the behavior of :func:`.orm.load_only` such that primary key

- Modified the behavior of :func:`.orm.load_only` such that primary key
columns are always added to the list of columns to be "undeferred";
otherwise, the ORM can't load the row's identity.   Apparently,
one can defer the mapped primary keys and the ORM will fail, that
hasn't been changed.  But as load_only is essentially saying
"defer all but X", it's more critical that PK cols not be part of this
deferral. fixes #3080