code: public String getHeader(String headerName) {
          List<String> headers = null;
          for (String h : headers().keySet()) {
              if (headerName.equalsIgnoreCase(h)) {
                  headers = headers().get(h);
                  break;
              }
          }
          if (headers == null || headers.isEmpty()) {
              return null;
          }
          return headers.get(0);
      }
docs: Retrieves a single header.
  
  @param headerName the header name
  @return the value of the header. If the header has multiple value,
  the first one is returned. If the header has no value (is not specified in the request),
  {@literal null} is returned.