Naming conflict with include_flex_columns_from

[ageweke]

If you use include_flex_columns_from, and the flex_column on the target model has the same name as the association from the original model to the target model, you get an infinite recursion and hence SystemStackError. There is no easy solution to this, but we should raise a clear error instead of getting into infinite recursion.

(Note to myself to fix this soon.)