Stronger guarantees for quoting literals

[krlmlr]

Quoting NA values should always return a query that yields a NULL of the correct type. This might be especially challenging in MariaDB where the support for coercion is weak. Could perhaps be simulated by

SELECT x FROM (SELECT '' AS x, 0 AS y UNION ALL SELECT NULL AS x, 1 AS y) qq WHERE y = 1

for such backends.

From r-dbi/RPostgres#357.

Also, we should include at least date, time, and interval types.