完善测试

git-svn-id: http://code.alibabatech.com/svn/druid/trunk@821 b9813039-fb51-4c41-a8b9-e21c2acb5095

src/main/java/com/alibaba/druid/pool/PoolableConnection.java

@@ -845,7 +845,7 @@ public class PoolableConnection implements PooledConnection, Connection {
         holder.getStatementEventListeners().remove(listener);
     }
 
-    protected void checkOpen() throws SQLException {
+    public void checkOpen() throws SQLException {
         if (holder == null) {
             throw new SQLException("connection is closed");
         }