`outgoing_body_filter` not used

[kookster]

the outgoing_body_filter method is described and has a base implementation in hyper_resource.rb

  ## +outgoing_body_filter+ filters a hash of attribute keys and values
  ## on their way from a HyperResource to a request body.  Override this
  ## in a subclass of HyperResource to implement filters on outgoing data.
  def outgoing_body_filter(attr_hash)
    attr_hash
  end

https://github.com/gamache/hyperresource/blob/master/lib/hyper_resource.rb#L152-L157

But, it isn't called anywhere, so implementing it in a subclass does nothing.