fix exception below. when the connection is closed, need not to write da...

...ta to response

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org.webbitserver.WebbitException: null on [id: 0xab10ae9c, /10.1.224.127:56076 :> /10.13.80.154:9000]
        at org.webbitserver.WebbitException.fromException(WebbitException.java:36)
        at org.webbitserver.netty.NettyHttpResponse.error(NettyHttpResponse.java:165)
        at org.webbitserver.netty.NettyHttpChannelHandler$3.run(NettyHttpChannelHandler.java:94)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:662)
Caused by: java.nio.channels.ClosedChannelException
        at org.jboss.netty.channel.socket.nio.AbstractNioWorker.cleanUpWriteBuffer(AbstractNioWorker.java:381)
        at org.jboss.netty.channel.socket.nio.AbstractNioWorker.close(AbstractNioWorker.java:349)
        at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:93)
        at org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:107)
        at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312)
        at org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:88)
        at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178)