Motivation:
Gracefully respond on bad client request.
We have a set of errors produced by Android 7.1.1/7.1.2 clients where both headers `HttpHeaderNames.SEC_WEBSOCKET_VERSION` and `HttpHeaderNames.ORIGIN` are not present. Absence of the first headers leads to WebSocketServerHandshaker00 be applied as a handshaker. However, null 2nd header causes
```
java.lang.NullPointerException: value
io.netty.util.internal.ObjectUtil.checkNotNull(ObjectUtil.java:33)
io.netty.handler.codec.DefaultHeaders.addObject(DefaultHeaders.java:327)
io.netty.handler.codec.http.DefaultHttpHeaders.add(DefaultHttpHeaders.java:123)
io.netty.handler.codec.http.websocketx.WebSocketServerHandshaker00.newHandshakeResponse(WebSocketServerHandshaker00.java:162)
```
Which causes connection close with unclear reason.
Modification:
Added null-check, and in case of null an appropriate WebSocketHandshakeException is thrown.
Result:
In case of null `HttpHeaderNames.ORIGIN` header a WebSocketHandshakeException is caught by WebSocketServerProtocolHandler which sends a graceful `BAD_REQUEST`.
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