Allow soap_encoding in config to be used in soap/xml.rb

That way, in a Savon::Model I would be able to

    config.soap_encoding = "UTF-16"

as seen previous in [issue 234](https://github.com/savonrb/savon/issues/234) instead of having to do that on each request inside the model    

Not sure if this is the best way to do it, but I gave it a shot...

Let me know if I can help, best,