I can very easily see users wanting to use `data` as a collection or event property. Its not currently an issue, but could very much limit supporting bc issues if things change.

That collision could affect us if we have to inspect the values being passed through to support bc issues.  Since there is no mention of that property in the docs (because its guzzle specific), I think the naming needs to be more self explanatory and unique.  

I liked having it effectively namespaced under `keen_io` - if you disagree, what about `request_body` or something similar?