Move System::httpExit to BaseModule->httpExit

- This will ensure headers set in BaseModule->run will be carried in httpExit scenarios
- Deprecate httpExit() method in Core\System

src/Module/Feed.php

@@ -71,6 +71,6 @@ class Feed extends BaseModule
 
 		$feed = ProtocolFeed::atom($owner, $last_update, 10, $type);
 
-		System::httpExit($feed, Response::TYPE_ATOM);
+		$this->httpExit($feed, Response::TYPE_ATOM);
 	}
 }