code: @Nonnull
    public ApiFuture<WriteResult> set(@Nonnull Object pojo) {
      WriteBatch writeBatch = firestore.batch();
      return extractFirst(writeBatch.set(this, pojo).commit());
    }
docs: Overwrites the document referred to by this DocumentReference. If no document exists yet, it
  will be created. If a document already exists, it will be overwritten.
  
  @param pojo The POJO that will be used to populate the document contents.
  @return An ApiFuture that will be resolved when the write finishes.