close() method of BGPSessionImpl should set the error subcode

I got null pointer exception when the close() method of BGPSessionImpl was called.
I find that the root cause is that the error subcode of BGPError.CEASE is not set.

Please set the errorSubcode for BGPError.CEASE in the close() method.

FYI:
this.sendMessage(new NotifyBuilder().setErrorCode(BGPError.CEASE.getCode()).setErrorSubcode((short)0).build());