Fix #220 continue to override isClosed() ... we had removed it but it should reflect HikariCP's understanding of the closed state rather than delegating to the driver.

hikaricp-common/src/main/java/com/zaxxer/hikari/proxy/ConnectionProxy.java

@@ -207,6 +207,13 @@ public abstract class ConnectionProxy implements IHikariConnectionProxy
       }
    }
 
+   /** {@inheritDoc} */
+   @Override
+   public boolean isClosed() throws SQLException
+   {
+      return (delegate == ClosedConnection.CLOSED_CONNECTION);
+   }
+
    /** {@inheritDoc} */
    @Override
    public Statement createStatement() throws SQLException