Fix project_id field in MergeRequest API

lib/api/entities.rb

@@ -107,7 +107,8 @@ module API
     end
     class MergeRequest < Grape::Entity
-      expose :id, :target_branch, :source_branch, :project_id, :title, :state
+      expose :id, :target_branch, :source_branch, :title, :state
+      expose :target_project_id, as: :project_id
       expose :author, :assignee, using: Entities::UserBasic
     end