Missing 'data' from webhook fulfillment

[MargaretaKusan]

I cannot access the data property from the fulfillment in the webhook response. It only contains 'speech' and 'messages'. I forked the repo and added a parameter 'data' to the AIFulfillmentResponse.h and AIFulfillmentResponse.m accordingly. If you agree, I will submit the pull request.

@property(nonatomic, copy, readonly) NSDictionary *data;