Description:
------------
MYSQLI_UNIQUE_KEY_FLAG aren't being indicated for columns that have UNIQUE indexes.

Reproduce code:
---------------
mysqli_query($db->connection,
             'CREATE TABLE phptest_fk (a VARCHAR(5),' .
             ' UNIQUE (a))');

$r = mysqli_query($db->connection,
                  'SELECT a FROM phptest_fk');

$tmp = mysqli_fetch_field($r);
echo 'MYSQLI_UNIQUE_KEY_FLAG? ';
if ($tmp->flags & MYSQLI_UNIQUE_KEY_FLAG) {
    echo "yes\n";
} else {
    echo "no\n";
}

mysqli_query($db->connection,
             'DROP TABLE phptest_fk');


Expected result:
----------------
MYSQLI_UNIQUE_KEY_FLAG? yes

Actual result:
--------------
MYSQLI_UNIQUE_KEY_FLAG? no