Fix EE not returning issue weights for boards

We use `if` in the serializer because weights is an EE only feature.

Add specs in addition to the feature spec to make we return weight in
the JSON response.

app/serializers/issue_board_entity.rb

@@ -11,7 +11,7 @@ class IssueBoardEntity < Grape::Entity
   expose :due_date
   expose :project_id
   expose :relative_position
-  expose :weight, if: -> (*) { respond_to?(:weight) }
+  expose :weight, if: -> (instance, _) { instance.respond_to?(:weight) }
   expose :time_estimate
 
   expose :project do |issue|

ee/spec/controllers/boards/issues_controller_spec.rb

@@ -49,6 +49,15 @@ describe Boards::IssuesController do
           expect(parsed_response['issues'].length).to eq 2
           expect(development.issues.map(&:relative_position)).not_to include(nil)
         end
+
+        it 'returns issues with weight attribute' do
+          create(:labeled_issue, project: project_1, labels: [planning], weight: 3)
+
+          list_issues user: user, board: board, list: list1
+
+          parsed_response = JSON.parse(response.body)
+          expect(parsed_response['issues'].first["weight"]).to eq 3
+        end
       end
 
       context 'with invalid list id' do