Correct the root context path used with Undertow

Undertow, like Tomcat, uses "" for the context path of the root
context. Previously, the Undertow deployment was being configured with
"/" for the root context. This was leading to a silent failure in
AsyncContextImpl.dispatch when it failed to look up the deployment
manager for the current request.

This commit updates UndertowEmbeddedServletContainerFactory to use the
correct context path (an empty String) for the root context.

Fixes gh-2365

spring-boot-samples/spring-boot-sample-undertow/src/main/java/sample/undertow/web/SampleController.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,23 +16,37 @@
 
 package sample.undertow.web;
 
+import java.util.concurrent.Callable;
+
 import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.RequestMapping;
-import org.springframework.web.bind.annotation.ResponseBody;
+import org.springframework.web.bind.annotation.RestController;
 
 import sample.undertow.service.HelloWorldService;
 
-@Controller
+@RestController
 public class SampleController {
 
 	@Autowired
 	private HelloWorldService helloWorldService;
 
 	@RequestMapping("/")
-	@ResponseBody
 	public String helloWorld() {
 		return this.helloWorldService.getHelloMessage();
 	}
 
+	@RequestMapping("/async")
+	public Callable<String> helloWorldAsync() {
+		return new Callable<String>() {
+
+			@Override
+			public String call() throws Exception {
+				return "async: "
+						+ SampleController.this.helloWorldService.getHelloMessage();
+			}
+
+		};
+
+	}
+
 }