Expose some attributes only when they're required.

This was breaking the spec spec/features/merge_requests/created_from_fork_spec.rb
which was trying to visit a MergeRequest after the source project was
deleted.

app/controllers/projects/merge_requests_controller.rb

@@ -62,7 +62,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       format.html { define_discussion_vars }
 
       format.json do
-        render json: MergeRequestSerializer.new.represent(@merge_request)
+        render json: MergeRequestSerializer.new.represent(@merge_request, type: :full)
       end
 
       format.patch  do

app/serializers/merge_request_entity.rb

@@ -9,7 +9,7 @@ class MergeRequestEntity < IssuableEntity
   expose :merge_user_id
   expose :merge_when_build_succeeds
   expose :rebase_commit_sha
-  expose :rebase_in_progress?
+  expose :rebase_in_progress?, if: { type: :full }
   expose :source_branch
   expose :source_project_id
   expose :target_branch