NH-3559 - UnionSubclassEntityPersister does not quote column names

UnionSubclassEntityPersister does not quote column names when "hbm2ddl.keywords" is enabled. This for example makes NHibernate generated queries fail when quering class with reserved keyword as property name (for example 'User', 'Table', etc.)

Reported at https://nhibernate.jira.com/browse/NH-3559