[CHANGE] Service##messages= removed

The private method was removed as useless. Use messages#concat
to add messages to array:

  def on_item_not_found(*, messages)
    self.messages.concat messages
  end

In any case it's impossible to use both the @messages (it is not the array),
or self.messages += (#messages= was private and cannot be used with self).

== Changes

* Hexx::Helpers::Messages#messages= removed
* Hexx::Service#messages= not defined any more