docs: public String getHeaderField(String fieldName) {
          // headers map is null for all regular response calls except when made as a batch request
          if (this.headers == null) {
              if (this.connection != null) {
                  return this.connection.getHeaderField(fieldName);
              } else {
                  return null;
              }
          } else {
              return this.headers.get(fieldName);
          }
      }