Fixes #37

In generated code, relying on batch.set(reference, json, options); for batchSet implementation is wrong because we pass a DocumentReference<T> with T a mapping class as a reference with a json data. Cloud_firestore then try to apply the converter but data is already in json. See write_batch.dart from cloud_firestore, line 56.

batch.set(reference, json, options);

Should be replaced with :

    final castedReference = reference.withConverter<Map<String, dynamic>>(
      fromFirestore: (snapshot, options) => throw UnimplementedError(),
      toFirestore: (value, options) => value,
    );

    batch.set(castedReference, json, options);

Like it's done for set method. Same for transaction.

• Tests pass
• Appropriate changes to README are included in PR