Prefixed fields in traits

[RazorMeister]

I've encountered a problem while implementing this package because in request I have field named additional. And this field is already used in AppendableData trait. (I know that I can Map this field).

So I suggest to prefix all fields from used traits with for example underscore to avoid such problems. (like additional -> _additional)

[rubenvanassche]

You can always send a PR in for this, the problem is that we probably going to need to release a major version since this kinda breaks the API. Some properties can be overwritten on a data object and they will not be used anymore when we prefix them with _.