code: public function injectRobots ()
  	{
  		$headers = \Craft::$app->getResponse()->getHeaders();
  
  		// If devMode always noindex
  		if (\Craft::$app->config->general->devMode)
  		{
  			$headers->set('x-robots-tag', 'none, noimageindex');
  			return;
  		}
  
  		list($field, $element) = $this->_getElementAndSeoFields();
  
  		// Robots
  		$robots = $field->robots;
  
  		if ($robots !== null)
  			$headers->set('x-robots-tag', $robots);
  
  		// Get Expiry Date
  		/** @var \DateTime $expiry */
  		if (isset($element->expiryDate))
  			$expiry = $element->expiryDate->format(\DATE_RFC850);
  		else
  			$expiry = null;
  
  		// If we've got an expiry time, add an additional header
  		if ($expiry)
  			$headers->add('x-robots-tag', 'unavailable_after: ' . $expiry);
  	}
docs: Adds the `X-Robots-Tag` header to the request if needed.