As written, the code would only fail the sanity check if none of the
columns returned by the server were of the expected type, but we want
it to fail if even one column is not of the expected type.

Discussion: http://postgr.es/m/CA+TgmoYuY5zW7JEs+1hSS1D=V5K8h1SQuESrq=bMNeo0B71Sfw@mail.gmail.com