code: private static void addHeaderIfNotNull(Request<?> request, String header, String value) {
          if (value != null) {
              request.addHeader(header, value);
          }
      }
docs: Adds the specified header to the specified request, if the header value
  is not null.
  
  @param request
  The request to add the header to.
  @param header
  The header name.
  @param value
  The header value.