Fix spec by matching entity with schema

ee/app/serializers/epic_entity.rb

@@ -10,6 +10,7 @@ class EpicEntity < IssuableEntity
   expose :start_date
   expose :start_date_is_fixed?, as: :start_date_is_fixed
   expose :start_date_fixed, :start_date_from_milestones
+  expose :end_date # @deprecated
   expose :end_date, as: :due_date
   expose :due_date_is_fixed?, as: :due_date_is_fixed
   expose :due_date_fixed, :due_date_from_milestones

ee/spec/fixtures/api/schemas/entities/epic.json

@@ -12,7 +12,14 @@
     "milestone_id": { "type": ["string", "null"] },
     "state": { "type": "string" },
     "start_date": { "type": ["date", "null"] },
+    "start_date_fixed": { "type": ["date", "null"] },
+    "start_date_is_fixed": { "type": "boolean" },
+    "start_date_from_milestones": { "type": ["date", "null"] },
     "end_date": { "type": ["date", "null"] },
+    "due_date": { "type": ["date", "null"] },
+    "due_date_fixed": { "type": ["date", "null"] },
+    "due_date_from_milestones": { "type": ["date", "null"] },
+    "due_date_is_fixed": { "type": "boolean" },
     "updated_by_id": { "type": ["string", "null"] },
     "created_at": { "type": "string" },
     "updated_at": { "type": "string" },