Store should merge fields if both modelClass and fields configs are defined

[chuckn0rris]

Forum post

Our docs says that store does merge fields with model's fields, but now it works only if modelClass is not provided. Should be possible to provide both

// now this won't merge fields, `fields` just ignored.
let store = new Store({
   data: [order],
   fields : [{ name: "BuyerName", type: "string", dataSource: "Buyer.Name" }],
   modelClass: Order
});

[isglass]

It already merges fields, problem is that user is supplying a record and not data. Will clarify in docs