Fix id override

[beauby]

Currently, the id attribute is handled in a special way, which leads to the impossibility to override it before the attribute :id call.
I believe this special treatment is just legacy code. @bf4, @joaomdmoura?

[beauby]

Ref #1188.

[beauby]

Actually, the id check was there because there used to be an id method defined on the Serializer class as follows:

def id
  object.id if object
end

which was there to make sure the JsonApi adapter could get the id. Now the logic has been (rightfully) moved to the JsonApi adapter, so the default id method and the check are not necessary anymore.

[NullVoxPopuli]

👍