Statement preparation has empty return value args

This patch is more a question than a solution. It includes a test that demonstrates that the `rval` field of the `*queryInfo` instance return as result of statement preparation is indeed empty. Seeing as the statement that this query info is based on returns two columns, that could be interpreted as being misleading. Maybe the rval field has nothing to do with the query result columns. But I think we need to clarify this if we intend to expose this data externally.