Allow nested polymorphic has_many associations

Allows to nested field type to handle polymorphic has_many associations, thus fixing #1338.

The only problem was in the :inverse_of presence checking, where the FormBuilder tried to access `field.associated_model_config` without checking if it was a 1..1 or 1..N association.

I don't really know where to write my test for that, if someone can give me some insights on where to write it I'll be pleased to add it.

I wait for any advice to enhance this request.