Assigning null to 'id' attribute causes PostgreSQL to throw Not Null violation

[dor-denis]

When using Factory::make or Factory::create null value is being assigned to 'id' attribute of object. When the query is being generated it looks like INSERT INTO ('id' ...) VALUES (null, ...) which causes Not Null violation.
I have to add
if($key == 'id') continue;
to line 254 of Factory.php to make my tests work.