[MM-37755] Idiomatic naming (SMTP, TLS, TCP, XML, CSS, HTML, HTTP) (#18103)

app/plugin_requests.go

@@ -166,7 +166,7 @@ func (s *Server) servePluginRequest(w http.ResponseWriter, r *http.Request, hand
 			}
 
 			// ToDo(DSchalla) 2019/01/04: Remove after deprecation period and only allow CSRF Header (MM-13657)
-			if r.Header.Get(model.HeaderRequestedWith) == model.HeaderRequestedWithXml && !csrfCheckPassed {
+			if r.Header.Get(model.HeaderRequestedWith) == model.HeaderRequestedWithXML && !csrfCheckPassed {
 				csrfErrorMessage := "CSRF Check failed for request - Please migrate your plugin to either send a CSRF Header or Form Field, XMLHttpRequest is deprecated"
 				sid := ""
 				userID := ""