New behavior for INSERT queries with RETURNING clause
Investigating #1522, it occurred to me that INSERT queries with non-default
RETURNING clauses (postgres-only feature) should always return a cursor
object. Previously, if executing a single-row INSERT query, the last-inserted
row ID would be returned, regardless of what was specified by the RETURNING
clause.
This change only affects INSERT queries with non-default RETURNING clauses and
will cause a cursor to be returned, as opposed to the last-inserted row ID.