Require POST method for '/checkframeoptionsheader'

This disables CORS for this path. It should not be usable by external
websites.

src/main/java/de/thm/arsnova/controller/WelcomeController.java

@@ -63,7 +63,7 @@ public class WelcomeController extends AbstractController {
 		return new HashMap<String, Object>();
 	}
 
-	@RequestMapping(value = "/checkframeoptionsheader", method = RequestMethod.GET)
+	@RequestMapping(value = "/checkframeoptionsheader", method = RequestMethod.POST)
 	@ResponseStatus(HttpStatus.OK)
 	public void checkFrameOptionsHeader(
 			@RequestParam(required = true) final String url,