Fix our monkey-patched `Backbone.Model.set()`
Fix our monkey-patched `Backbone.Model.set()`

Our `Backbone.Model.set()` replaces empty strings with `null` for JSON-happiness, but it was out of date with the arguments allowed by `Backbone.Model.set()` proper. Update, and drop a note about keeping in sync.

Teino1978-Corp/Teino1978-Corp-documentcloud,Teino1978-Corp/Teino1978-Corp-documentcloud,ivarvong/documentcloud,documentcloud/documentcloud,dannguyen/documentcloud,ivarvong/documentcloud,monofox/documentcloud,monofox/documentcloud,Teino1978-Corp/Teino1978-Corp-documentcloud,gunjanmodi/documentcloud,ivarvong/documentcloud,dannguyen/documentcloud,monofox/documentcloud,gunjanmodi/documentcloud,gunjanmodi/documentcloud,gunjanmodi/documentcloud,Teino1978-Corp/Teino1978-Corp-documentcloud,dannguyen/documentcloud,ivarvong/documentcloud,dannguyen/documentcloud,documentcloud/documentcloud,monofox/documentcloud,documentcloud/documentcloud,documentcloud/documentcloud