how do I alter key format when deserializing

[bsylvain]

I want to replace dash by underscore in the keys when I deserialize. I tried to alter the Deserializable resource in an initializer :
JSONAPI::Deserializable::Resource.key_formatter = proc {|k| k.to_s.underscore }

and it does not seems to be working. I end up with always the same warning:
Unpermitted parameters: :dasherized-param

It seems there is a way to alter key at deserialization, how can I do that?