Allow the default value of `include_xyz?` to be specified easily

[GavinJoyce]

It's currently possible to do serializer.as_json(model, { include_description?: false }) which will exclude the description attribute. The default is to include the description.

It would be nice to be able to easily set the default to not include the attribute. Something like:

class PersonSerializer
    include RestPack::Serializer
    attributes :id, :name, { description: { include_by_default: false } }
end

serializer.as_json(model) would exclude the description serializer.as_json(model, { include_description?: false }) would exclude the description serializer.as_json(model, { include_description?: true }) would include the description

[GavinJoyce]

@eugeneius, how about:

class PersonSerializer
    include RestPack::Serializer

    attributes :id, :name
    optional_attributes :description
end

or

class PersonSerializer
    include RestPack::Serializer

    attributes :id, :name
    optional :description
end

[eugeneius]

Adding them separately definitely reads better than flagging them inline as part of attributes :+1:

[eugeneius]

What about using a similar API to mutations:

class PersonSerializer
  include RestPack::Serializer

  attributes :id, :name

  optional do
    attributes :description
  end
end

[GavinJoyce]

That's nice however I think if I added support for that, the main API should become:

class PersonSerializer
  include RestPack::Serializer

  required do
    attributes :id, :name
  end

  optional do
    attributes :description
  end
end

I think I'll keep it simple for now, I've almost got a PR ready