Make serializers serializable, step 1.

[bf4]

I think this is a first step in a good idea...

This comes up not infrequently, especially as it was an interface in previous releases.

Now, a serializer responds to serializable_hash, as_json, to_json, to_h, and to_hash as is expected from experience, and sorta makes sense, as well. (to_h and to_hash are sentinels Ruby uses for generating json etc)

e.g. #1487

Obviously it needs tests, but I wanted to just get the idea out there.

re step 2, it's non-trivial.. I futzed a bit in bf4/active_model_serializers@serializable_serializer...serializable_serializers and bf4/active_model_serializers@serializable_serializer...serializable_serializers_take1

ref:

• https://github.com/rails/rails/blob/v5.0.0.beta2/activemodel/lib/active_model/serialization.rb
• https://github.com/rails/rails/blob/v5.0.0.beta2/activemodel/lib/active_model/serializers/json.rb
• https://github.com/rails/rails/blob/v5.0.0.beta2/activerecord/lib/active_record/serialization.rb#L11-L17

[bf4]

and include ActiveModel::Serializers::JSON. So that the below is true:

this is really a new feature that probably be implemented

[bf4]

ref: #1380