code: @Override public final String getHeaderField(String fieldName) {
      try {
        return fieldName == null
            ? StatusLine.get(getResponse().getResponse()).toString()
            : getHeaders().get(fieldName);
      } catch (IOException e) {
        return null;
      }
    }
docs: Returns the value of the field corresponding to the {@code fieldName}, or
  null if there is no such field. If the field has multiple values, the
  last value is returned.