a way to fetch data when building from form_params

[nduitz]

I don't know if this is the best way of dealing with it.

The problem of the previous code is, that there was no way of matching existing data to form_params for polymorphic_embeds_many.

The reason is simple. form params are typically string keys and are passed as such to the changeset.
The previous code only did look at :id when trying to find existing data to apply changes to.

Now the cast method also checks for params[persistent_id] which is automatically added by polymorphic_inputs_for
This way data that isn't send forth and back by the form will persist in changesets.

[nduitz]

This won't work when adding/removing inputs dynamically using sort_param and drop_param. I'll take a closer look and implement more tests

[nduitz]

this seems to be unnecessary with this patch:
#132

Therefore I will close it.