Allow to change model name for polymorphic queries

The `hasmany` association always use the name of the model as the value against
the `polymorphic_key`.

When using STI (single-table inheritance) you might want to always use the parent model.

This change allows just that.
Merge branch 'hasmany-polymoprhic-sti'

* hasmany-polymoprhic-sti:
  Allow to change model name for polymorphic queries