[CI] HttpReadWriteHandlerTests.testSuccessfulDecodeHttpRequest LEAK

[henningandersen]

HttpReadWriteHandlerTests.testSuccessfulDecodeHttpRequest failed on my PR build here: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+pull-request-1/13279/testReport/org.elasticsearch.http.nio/HttpReadWriteHandlerTests/testSuccessfulDecodeHttpRequest/

This reproduces easily by running test class in IntelliJ until failure (just running the method alone is not enough). Somehow every 19th run fails (but not on the same test), probably GC related (no evidence for that statement).

java.lang.AssertionError: 
Expected: an empty collection
     but: <[LEAK: ByteBuf.release() was not called before it's garbage-collected. See http://netty.io/wiki/reference-counted-objects.html for more information.
Recent access records: 
#1:
	io.netty.buffer.AdvancedLeakAwareByteBuf.nioBuffers(AdvancedLeakAwareByteBuf.java:724)
	org.elasticsearch.http.nio.NettyAdaptor$1.write(NettyAdaptor.java:58)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:721)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:713)
	io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:796)
	io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:706)
	io.netty.handler.codec.MessageToMessageEncoder.write(MessageToMessageEncoder.java:112)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:721)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:713)
	io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:796)
	io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:706)
	io.netty.handler.codec.MessageToMessageEncoder.write(MessageToMessageEncoder.java:112)
	io.netty.handler.codec.MessageToMessageCodec.write(MessageToMessageCodec.java:116)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:721)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:713)
	io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:796)
	io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:706)
	org.elasticsearch.http.nio.NioHttpPipeliningHandler.write(NioHttpPipeliningHandler.java:71)
	io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:721)

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