Expose remove_source_branch boolean/null

app/serializers/merge_request_entity.rb

@@ -39,6 +39,7 @@ class MergeRequestEntity < IssuableEntity
   expose :commits_count
   expose :cannot_be_merged?, as: :has_conflicts
   expose :can_be_merged?, as: :can_be_merged
+  expose :remove_source_branch?, as: :remove_source_branch
 
   expose :project_archived do |merge_request|
     merge_request.project.archived?

spec/fixtures/api/schemas/entities/merge_request.json

@@ -92,7 +92,8 @@
     "diverged_commits_count": { "type": "integer" },
     "commit_change_content_path": { "type": "string" },
     "remove_wip_path": { "type": "string" },
-    "commits_count": { "type": "integer" }
+    "commits_count": { "type": "integer" },
+    "remove_source_branch": { "type": ["boolean", "null"] }
   },
   "additionalProperties": false
 }