docs: public String getResponseBodyAsString(int maxlen) throws IOException {
          if (maxlen < 0) throw new IllegalArgumentException("maxlen must be positive");
          byte[] rawdata = null;
          if (responseAvailable()) {
              rawdata = getResponseBody(maxlen);
          }
          if (rawdata != null) {
              return EncodingUtil.getString(rawdata, getResponseCharSet());
          } else {
              return null;
          }
      }