Fields filtering with include

Version 0.9.12

When using fields[events]=name + include=establishment the key relationships disappear from the response hash.

Basic association is events belongs_to establishment

Issue is coming from here:

# JSONAPI::ResourceSerializer#supplying_relationship_fields

def supplying_relationship_fields(resource_klass)
        @_supplying_relationship_fields.fetch resource_klass do
          relationships = Set.new(resource_klass._relationships.keys.map(&:to_sym))
          cur = resource_klass
          while cur != JSONAPI::Resource
            if @fields.has_key?(cur._type)
              # here... 
              # It always empty the Set
              #relationships &= @fields[cur._type]
              break
            end
            cur = cur.superclass
          end
          @_supplying_relationship_fields[resource_klass] = relationships
        end
      end

Commenting relationships &= @fields[cur._type] solves this issue. @DavidMikeSimon could you describe your mind here? I know it was a loooong time ago but your help might help to correctly fix this issue.

Start in lib/jsonapi/resource_serializer.rb at JSONAPI::ResourceSerializer#supplying_relationship_fields and reproduce the fields[events]=name plus include=establishment case described in the issue. Verify the response hash retains the relationships key while still applying field filtering; the intended behavior should be confirmed before changing the set intersection.