code: protected HttpResponse executeUsingHead() throws IOException {
      Preconditions.checkArgument(uploader == null);
      HttpResponse response = executeUnparsed(true);
      response.ignore();
      return response;
    }
docs: Sends the metadata request using HEAD to the server and returns the raw metadata
  {@link HttpResponse} for the response headers.
  
  <p>
  Only supported when the original request method is GET. The response content is assumed to be
  empty and ignored. Calls {@link HttpResponse#ignore()} so there is no need to disconnect the
  response. Example usage:
  </p>
  
  <pre>
  HttpResponse response = request.executeUsingHead();
  look at response.getHeaders()
  </pre>
  
  <p>
  Subclasses may override by calling the super implementation.
  </p>
  
  @return the {@link HttpResponse}