Allow an ID of object to be customized directly on the serializer by
passing a block to `set_id` as opposed to only through a model property.
We already allow for attributes that do not have a model property of the
same name to be customized directly on the serializer using a block.
This customization can be useful in situation in which you have
different classes being serialized using the same serializer. For
example, if we have `HorrorMovie`, `ComedyMovie` and `DramaMovie` using
the same `MovieSerializer`, we can unify their IDs using
```
class MovieSerializer
include FastJsonapi::ObjectSerializer
attributes :name, :year
set_id do |record|
"#{record.name.downcase}-#{record.id}"
end
```
which is preferable to creating a `#serialized_id` method in every model
that will use `MovieSerializer` to encapsulate the customization.
Closes#315
9fa26fa588