code: public String getHeaderFieldKey(int pIndex) {
          // TODO: getInputStream() first, to make sure we have header fields
          if (pIndex >= responseHeaders.length) {
              return null;
          }
          String field = responseHeaders[pIndex];
  
          if (StringUtil.isEmpty(field)) {
              return null;
          }
          int idx = field.indexOf(':');
  
          return StringUtil.toLowerCase(((idx > 0)
                  ? field.substring(0, idx)
                  : field));
      }
docs: Returns the key for the {@code n}<sup>th</sup> header field.
  
  @param pIndex an index.
  @return the key for the {@code n}<sup>th</sup> header field,
  or {@code null} if there are fewer than {@code n}
  fields.