code: public Observable<AVUser> signUpInBackground() {
      JSONObject paramData = generateChangedParam();
      logger.d("signup param: " + paramData.toJSONString());
      return PaasClient.getStorageClient().signUp(paramData).map(new Function<AVUser, AVUser>() {
        @Override
        public AVUser apply(AVUser avUser) throws Exception {
          AVUser.this.mergeRawData(avUser);
          return AVUser.this;
        }
      });
    }
docs: sign up in background.
  @return