One-to-one in mapping by code does not allow setting the Class. Discussed in nhusers: https://groups.google.com/forum/#!topic/nhusers/-VdilcvwGBk
Pull request: https://github.com/nhibernate/nhibernate-core/pull/297
Merged in 96ad67b9b619c75ec2872e1551c924f77cdd5ac9.