修正isWrapFor && unwrap

git-svn-id: http://code.alibabatech.com/svn/druid/trunk@478 b9813039-fb51-4c41-a8b9-e21c2acb5095

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

@@ -447,15 +447,14 @@ public class PoolableConnection implements PooledConnection, Connection {
 
     @Override
     public <T> T unwrap(Class<T> iface) throws SQLException {
-        checkOpen();
-
+        if (conn == null) {
+            return null;
+        }
         return conn.unwrap(iface);
     }
 
     @Override
     public boolean isWrapperFor(Class<?> iface) throws SQLException {
-        checkOpen();
-
         return conn.isWrapperFor(iface);
     }