class Protocol::HTTP::Body::Completable

@parameter block [Proc] the callback to invoke when the body is closed.
@parameter message [Request | Response] the message body.

Wrap a message body with a callback. If the body is empty, the callback is invoked immediately.

def self.wrap(message, &block)
	if body = message&.body and !body.empty?
		message.body = self.new(message.body, block)
	else
		yield
	end
end