DataExtension null fields

[barberj]

reported by @jmcnevin

It would be nice to use the latest version of Savon, and by extension, the latest version of Gyoku.

I've run into an issue with trying to save DataExtension rows containing null values. Looking at the ET documentation, it would appear that I need to shape the SOAP messages slightly differently in this case:

https://help.exacttarget.com/en-US/technical_library/web_service_guide/technical_articles/updating_a_data_extension_object_with_a_null_field_value/

However, the version of gyoku that savon 2.0 wants (~>1.0.0) is broken in regards to attaching attributes to arrays, so it's making this a little difficult.

first referenced at salesforce-marketingcloud/FuelSDK-Ruby#7 (comment)