Fix index.html root path forwarding

Fix automatic '/' -> 'index.html' forwarding to also work when a
`server.servlet-path` property is set.

Fixes gh-2351

Author: philwebb

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.java

@@ -281,7 +281,7 @@ private void addStaticIndexHtmlViewControllers(ViewControllerRegistry registry)
 						// Ignore
 					}
 					// Use forward: prefix so that no view resolution is done
-					registry.addViewController("/").setViewName("forward:/index.html");
+					registry.addViewController("/").setViewName("forward:index.html");
 					return;
 				}
 			}