NH-3455 fix

Bug fix for https://nhibernate.jira.com/browse/NH-3455

This is ready for code review. Please let me know if there's anything I might be overlooking, as this is my first contribution and I'm extremely new to this codebase.

Questions:
* Does this mean that `projection.GetColumnAliases(propertyName, 0);` is buggy, or was it just being misused?