Fix configuration of session ID generator on Tomcat 7

See gh-6679

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

@@ -19,6 +19,7 @@ package org.springframework.boot.context.embedded.tomcat;
 import org.apache.catalina.Container;
 import org.apache.catalina.Manager;
 import org.apache.catalina.core.StandardContext;
+import org.apache.catalina.session.ManagerBase;
 
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
@@ -52,8 +53,8 @@ class TomcatEmbeddedContext extends StandardContext {
 
 	@Override
 	public void setManager(Manager manager) {
-		if (manager != null) {
-			manager.setSessionIdGenerator(new LazySessionIdGenerator());
+		if (manager instanceof ManagerBase) {
+			((ManagerBase) manager).setSessionIdGenerator(new LazySessionIdGenerator());
 		}
 		super.setManager(manager);
 	}