Add Vary header in case of content negotiation

Sometimes we return different content depending on whether JSON, XML or
HTML was requested in the Accept request header. The Vary response
header should list that header in these cases, to allow caching
frameworks to determine what to cache.

src/Module/Item/Display.php

@@ -162,6 +162,8 @@ class Display extends BaseModule
 
 		$output .= $this->getDisplayData($item);
 
+		header('Vary: Accept', false);
+
 		return $output;
 	}