Reinstate Tomcat’s old default behaviour for context root redirects

Prior to Tomcat 8.0.30, its default behaviour was for the mapper to
redirect a request for context-root to context-root/. This redirection
was switched off by default in 8.0.30 by default. This has proven
to be problematic and Tomcat 8.0.31 will reinstate the old default.

While we are waiting for Tomcat 8.0.31 to be released, this commit
explicitly sets the default back to what it was in 8.0.29 and earlier
and what it will be in 8.0.31.

Closes gh-4937

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainerFactory.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -169,6 +169,7 @@ public class TomcatEmbeddedServletContainerFactory
 						: ClassUtils.getDefaultClassLoader());
 		try {
 			context.setUseRelativeRedirects(false);
+			context.setMapperContextRootRedirectEnabled(true);
 		}
 		catch (NoSuchMethodError ex) {
 			// Tomcat is < 8.0.30. Continue