doctrine/doctrine2 — pull request #508 by mnapoli (2012-11-06 14:07:45+00:00)
Title: Fix and test for DDC-2073

Fix and test for [DDC-2073](http://www.doctrine-project.org/jira/browse/DDC-2073).

Ticket extract:

> PersistentCollection::matching() always returns empty collection, when passed criteria's expression is created using ExpressionBuilder::isNull()
> This happens because expression created by ExpressionBuilder::isNull() is wrongly translated to SQL using '=' operator instead of 'IS NULL'.
> If the collection is already loaded, result is as expected.