Fix invalid cast on nullable custom type with Linq

[maca88]

This is a quick fix for the regression introduced by #2036, which restores the previous behavior for custom types. A proper fix where the user can control whether a cast is required can be made for the next release.

Fixes #2437

[hazzik]

I have a different idea how to fix it. I'll raise a PR to @maca88's fork.

The problem is upcast to nullable. IsCastRequired should check the unwrapped types because SQL does not care about nullability on the class level.

The correct fix, in my opinion is to include expression.Type.UnwrapIfNullable() != toType.UnwrapIfNullable() check after toType != typeof(object) here:

nhibernate-core/src/NHibernate/Linq/Visitors/HqlGeneratorExpressionVisitor.cs

Lines 606 to 611 in 26472ed

	private bool IsCastRequired(Expression expression, System.Type toType, out bool existType)
	{
	existType = false;
	return toType != typeof(object) &&
	IsCastRequired(ExpressionsHelper.GetType(_parameters, expression), TypeFactory.GetDefaultTypeFor(toType), out existType);
	}

[hazzik]

Actually. I've just pushed it here. Feel free to drop/revert the last commit if you disagree.