docs: 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));
      }