Use correct database type value instead of null in Postgresql provider.
Merge pull request #23 from janno-p/master

Use correct database type value instead of null in Postgresql provider.
use DBNull.Value isntead of null with option types
Option values should also be considered when parsing projection columns.
DBNull in result set should be treated same way as <null>.
Merge pull request #24 from janno-p/master

Fixes for using option values inside query expressions.
Merge branch 'master' of github.com:fsprojects/SQLProvider