Align default context path expectation with Undertow 1.1.3's behaviour

When configured with a context path of "", Undertow 1.1.3 changes the
context path to be "/". The change [1] was made to fix UNDERTOW-350
[2].

[1] https://github.com/undertow-io/undertow/commit/3db7707b8b34095d8d2fafecf94a10d101c62320
[2] https://issues.jboss.org/browse/UNDERTOW-350

See gh-2732

spring-boot/src/test/java/org/springframework/boot/context/embedded/undertow/UndertowEmbeddedServletContainerFactoryTests.java

@@ -146,7 +146,7 @@ public class UndertowEmbeddedServletContainerFactoryTests extends
 			}
 		});
 		this.container = factory.getEmbeddedServletContainer();
-		assertEquals("", contextPath.get());
+		assertEquals("/", contextPath.get());
 	}
 
 }