Respect sti_name

The identifier stored in the `inheritance_column` should always call `sti_name`. In some places `class.name` is being called instead, which is only a problem when `sti_name` is changed from the default. 

Did you notice the specifics? `StiPost` redefines `sti_name` for this use-case, and `StiComment` uses a proc to define `class_name`. So, fairly specific to using/testing a `sti_name` that's not the class name. I thought it might be best not to pollute the standard models.