Refactor savon-multipart for Savon2

Needs tests but the interface between savon-multipart and savon just became a
whole lot smaller.
Remove spec_helper

This isn't necessary anymore because Savon does not have a global configuration
manager in version 2. Instead, when you instantiate a client, you have an option
to pass in some options there, and you can also pass in options (that will take
precedence over the previous ones) when you call the #call method on a Savon::Client

Neat, and with way less global state than before. Either way, this spec helper isn't
needed anymore.
Update tests

Nothing crazy, just changed how we create the response