Fix #300 add isClosed() method to HikariDataSource

src/main/java/com/zaxxer/hikari/HikariDataSource.java

@@ -283,6 +283,16 @@ public class HikariDataSource extends HikariConfig implements DataSource, Closea
       }
    }
 
+   /**
+    * Determine whether the HikariDataSource has been closed.
+    *
+    * @return true if the HikariDataSource has been closed, false otherwise
+    */
+   public boolean isClosed()
+   {
+      return isShutdown;
+   }
+
    /**
     * Shutdown the DataSource and its associated pool.
     *