implements kotlin language feature

tutorials/spring-boot-restful/src/main/kotlin/org/jetbrains/kotlin/demo/GreetingController.kt

@@ -11,7 +11,7 @@ class GreetingController {
     val counter = AtomicLong()
 
     @GetMapping("/greeting")
-    fun greeting(@RequestParam(value = "name", defaultValue = "World") name: String) =
-            Greeting(counter.incrementAndGet(), "Hello, $name")
+    fun greeting(@RequestParam name: String?) =
+            Greeting(counter.incrementAndGet(), "Hello, ${name ?: "World"}")
 
-}
+}