Close-suppressing Connection proxy exposes target isClosed() state

Closes gh-24853

spring-jdbc/src/main/java/org/springframework/jdbc/datasource/SingleConnectionDataSource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -322,7 +322,7 @@ public class SingleConnectionDataSource extends DriverManagerDataSource implemen
 				return null;
 			}
 			else if (method.getName().equals("isClosed")) {
-				return false;
+				return this.target.isClosed();
 			}
 			else if (method.getName().equals("getTargetConnection")) {
 				// Handle getTargetConnection method: return underlying Connection.