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/test/java/sample/undertow/SampleUndertowApplicationTests.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.
@@ -34,6 +34,7 @@ import static org.junit.Assert.assertEquals;
  * Basic integration tests for demo application.
  *
  * @author Ivan Sopov
+ * @author Andy Wilkinson
  */
 @RunWith(SpringJUnit4ClassRunner.class)
 @SpringApplicationConfiguration(classes = SampleUndertowApplication.class)
@@ -47,10 +48,19 @@ public class SampleUndertowApplicationTests {
 
 	@Test
 	public void testHome() throws Exception {
+		assertOkResponse("/", "Hello World");
+	}
+
+	@Test
+	public void testAsync() throws Exception {
+		assertOkResponse("/async", "async: Hello World");
+	}
+
+	private void assertOkResponse(String path, String body) {
 		ResponseEntity<String> entity = new TestRestTemplate().getForEntity(
-				"http://localhost:" + this.port, String.class);
+				"http://localhost:" + this.port + path, String.class);
 		assertEquals(HttpStatus.OK, entity.getStatusCode());
-		assertEquals("Hello World", entity.getBody());
+		assertEquals(body, entity.getBody());
 	}
 
 }