make discovery work when user has SELECT on only some columns on a table

[cmerrick]

This change isn't compatible with Postgres<8.4, but we don't support versions prior to 9.3 anyway.

Tests confirm this change is backwards-compatible, as does this blurb from the postgres docs:

has_column_privilege checks whether a user can access a column in a particular way. Its argument possibilities are analogous to has_table_privilege, with the addition that the column can be specified either by name or attribute number. The desired access privilege type must evaluate to some combination of SELECT, INSERT, UPDATE, or REFERENCES. Note that having any of these privileges at the table level implicitly grants it for each column of the table.

[dmosorast]

LGTM. The "implicit" in the docs you linked had concerned me, but it looks like this query finds the columns if SELECT is only granted on the table, so I'm good to merge.