Upgrade from @Controller/@ResponseBody to @RestController

Resolves #22

complete/src/main/java/hello/GreetingController.java

@@ -1,21 +1,19 @@
 package hello;
 
 import java.util.concurrent.atomic.AtomicLong;
-import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestParam;
-import org.springframework.web.bind.annotation.ResponseBody;
+import org.springframework.web.bind.annotation.RestController;
 
-@Controller
+@RestController
 public class GreetingController {
 
     private static final String template = "Hello, %s!";
     private final AtomicLong counter = new AtomicLong();
 
     @RequestMapping("/greeting")
-    public @ResponseBody Greeting greeting(
-            @RequestParam(value="name", required=false, defaultValue="World") String name) {
+    public Greeting greeting(@RequestParam(value="name", required=false, defaultValue="World") String name) {
         return new Greeting(counter.incrementAndGet(),
                             String.format(template, name));
     }
-}
+}