Updated PgsqlAdapter boolean_to_string() for Postgres boolean values

Modified the boolean_to_string() method in the PgsqlAdapter class to
properly convert values that PHP considers a boolean, as well as
what PostgreSQL considers a boolean value.

E.g., PostgreSQL considers the values 'f', 'false', 'n', 'no', and
'off' as valid values for `false`, but PHP does not.  The original
implementation of this method would return `true` for one of these
values which is incorrect for a PostgreSQL database.

Also modified the base Connection class boolean_to_string() method
to first cast its `$value` parameter as a boolean before casting
it to a string.  This will account for PHP's loose type-casting
and make sure the method returns a consistent casted value.