Re-add the test for SELECT tag; I was wrong in my assumption.

Postgres generally does not emit a SELECT tag after a SELECT query, but
it does emit that tag after a CREATE TABLE x AS SELECT query.

Example:

postgres=# create table t as select 1;
SELECT 1

test/unit/client/result-metadata-tests.js

@@ -36,3 +36,4 @@ testForTag("INSERT 0 3", check(0, 3, "INSERT"));
 testForTag("INSERT 841 1", check(841, 1, "INSERT"));
 testForTag("DELETE 10", check(null, 10, "DELETE"));
 testForTag("UPDATE 11", check(null, 11, "UPDATE"));
+testForTag("SELECT 20", check(null, 20, "SELECT"));