In plpgsql, don't try to convert int2vector or oidvector to expanded array.

These types are storage-compatible with real arrays, but they don't support
toasting, so of course they can't support expansion either.

Per bug #14289 from Michael Overmeyer.  Back-patch to 9.5 where expanded
arrays were introduced.

Report: <20160818174414.1529.37913@wrigleys.postgresql.org>