Collect enum only if both collection options are blank

I want to pass a restricted enums hash. Example:

````
class Foo < ActiveRecord::Base
  enum status: { pending: 0, submitted: 1, approved: 2, active: 3, rejected: 4 }

  def self.restricted_statuses
    statuses.except :approved, :rejected
  end

end
````

In my app, depending on user role, you could see all statuses (`Foo.statuses`) or only specific ones (`Foo.restricted_statuses`). I was hoping to utilize simple_form's `:collection` key to do this:

`= f.input :status, collection: Foo.restricted_statuses`

however I noticed that enum_help does not use the `:collection` key.