code: public Request sendRequest(String requestData, RequestCallback callback) 
          throws RequestException {
          StringValidator.throwIfNull("callback", callback);
          return doSend(requestData, callback);
      }
docs: Sends an HTTP request based on the current builder configuration with the
  specified data and callback. If no request headers have been set, the
  header "Content-Type" will be used with a value of "text/plain;
  charset=utf-8". This method does not cache <code>requestData</code> or
  <code>callback</code>.
  
  @param requestData the data to send as part of the request
  @param callback the response handler to be notified when the request fails
  or completes
  @return a {@link Request} object that can be used to track the request
  @throws NullPointerException if <code>callback</code> <code>null</code>