Add epic_iid to issue API entity

(cherry picked from commit 0bf7fecdee5a200eeb4f27b4888a57ceee63bde9)

doc/api/issues.md

@@ -573,6 +573,18 @@ the `weight` parameter:
 }
 ```
 
+Users on GitLab [Ultimate](https://about.gitlab.com/pricing/) will additionally see
+the `epic_iid` property:
+
+```json
+{
+   "project_id" : 4,
+   "description" : "Omnis vero earum sunt corporis dolor et placeat.",
+   "epic_iid" : 42,
+   ...
+}
+```
+
 **Note**: `assignee` column is deprecated, now we show it as a single-sized array `assignees` to conform to the GitLab EE API.
 
 **Note**: The `closed_by` attribute was [introduced in GitLab 10.6][ce-17042]. This value will only be present for issues which were closed after GitLab 10.6 and when the user account that closed the issue still exists.

lib/api/entities.rb

@@ -978,7 +978,9 @@ module API
       expose :created_at
 
       def todo_target_class(target_type)
-        ::API::Entities.const_get(target_type)
+        # false as second argument prevents looking up in module hierarchy
+        # see also https://gitlab.com/gitlab-org/gitlab-ce/issues/59719
+        ::API::Entities.const_get(target_type, false)
       end
     end
 
@@ -1731,6 +1733,7 @@ API::Entities.prepend_if_ee('EE::API::Entities::Entities')
 ::API::Entities::Group.prepend_if_ee('EE::API::Entities::Group', with_descendants: true)
 ::API::Entities::GroupDetail.prepend_if_ee('EE::API::Entities::GroupDetail')
 ::API::Entities::IssueBasic.prepend_if_ee('EE::API::Entities::IssueBasic', with_descendants: true)
+::API::Entities::Issue.prepend_if_ee('EE::API::Entities::Issue')
 ::API::Entities::List.prepend_if_ee('EE::API::Entities::List')
 ::API::Entities::MergeRequestBasic.prepend_if_ee('EE::API::Entities::MergeRequestBasic', with_descendants: true)
 ::API::Entities::Namespace.prepend_if_ee('EE::API::Entities::Namespace')