Adjust permitted params filtering on merge scheduling

ActionController::Parameters#to_h filters all unpermitted params. And once we pass unpermitted params to MergeRequest#merge_async, it started getting params removed after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18701.

This MR ensures we pass a already permitted ActionController::Parameters to MergeRequest#merge_async.

closes #46903 (closed)