This can be reproduced once in ~3 runs. I tested on Linux X64, but looks not to be platform-related.

The test failed at different codes, but error stack is similar.

Please refer following test output:
May 05, 2016 7:28:29 PM java.net.http.PlainHttpConnection close
INFO: ERROR: Closing: PlainHttpConnection: HttpConnection: java.nio.channels.SocketChannel[connected local=/127.0.0.1:34669 remote=/127.0.0.1:55415]
Shutdown: java.io.IOException: Connection aborted
java.io.IOException: Connection aborted
at java.net.http.PlainHttpConnection.shutdown(java.httpclient@9-ea/PlainHttpConnection.java:231)
at java.net.http.PlainHttpConnection.asyncRead(java.httpclient@9-ea/PlainHttpConnection.java:252)
at java.net.http.PlainHttpConnection$ReadEvent.handle(java.httpclient@9-ea/PlainHttpConnection.java:332)
at java.net.http.HttpClientImpl$SelectorManager.handleEvent(java.httpclient@9-ea/HttpClientImpl.java:305)
at java.net.http.HttpClientImpl$SelectorManager.run(java.httpclient@9-ea/HttpClientImpl.java:275)
May 05, 2016 7:28:29 PM java.net.http.Http2Connection resetStream
INFO: ERROR: Resetting stream 1 with error code 8
java.nio.channels.ClosedChannelException
at sun.nio.ch.SocketChannelImpl.ensureWriteOpen(java.base@9-ea/SocketChannelImpl.java:272)
at sun.nio.ch.SocketChannelImpl.write(java.base@9-ea/SocketChannelImpl.java:463)
at java.net.http.PlainHttpConnection.write(java.httpclient@9-ea/PlainHttpConnection.java:184)
at java.net.http.AsyncSSLDelegate.upperWrite(java.httpclient@9-ea/AsyncSSLDelegate.java:202)
at java.net.http.Queue.putAll(java.httpclient@9-ea/Queue.java:58)
at java.net.http.AsyncSSLDelegate.write(java.httpclient@9-ea/AsyncSSLDelegate.java:166)
at java.net.http.AsyncSSLConnection.write(java.httpclient@9-ea/AsyncSSLConnection.java:85)
at java.net.http.Http2Connection.sendOneFrame(java.httpclient@9-ea/Http2Connection.java:763)
at java.net.http.Http2Connection.sendFrame(java.httpclient@9-ea/Http2Connection.java:739)
at java.net.http.Http2Connection.resetStream(java.httpclient@9-ea/Http2Connection.java:462)
at java.net.http.Stream.cancelImpl(java.httpclient@9-ea/Stream.java:630)
at java.net.http.Http2Connection.shutdown(java.httpclient@9-ea/Http2Connection.java:373)
at java.net.http.PlainHttpConnection.shutdown(java.httpclient@9-ea/PlainHttpConnection.java:231)
at java.net.http.PlainHttpConnection.asyncRead(java.httpclient@9-ea/PlainHttpConnection.java:252)
at java.net.http.PlainHttpConnection$ReadEvent.handle(java.httpclient@9-ea/PlainHttpConnection.java:332)
at java.net.http.HttpClientImpl$SelectorManager.handleEvent(java.httpclient@9-ea/HttpClientImpl.java:305)
at java.net.http.HttpClientImpl$SelectorManager.run(java.httpclient@9-ea/HttpClientImpl.java:275)
Closing java.net.http.Queue@5f758cf4
Closing java.net.http.Queue@7b6dd04d
Closing PlainHttpConnection: HttpConnection: java.nio.channels.SocketChannel[closed]
May 05, 2016 7:28:29 PM java.net.http.HttpClientImpl$SelectorManager run
INFO: ERROR: HttpClientImpl: fatal error: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(java.base@9-ea/HashMap.java:1489)
at java.util.HashMap$ValueIterator.next(java.base@9-ea/HashMap.java:1518)
at java.net.http.Http2Connection.shutdown(java.httpclient@9-ea/Http2Connection.java:372)
at java.net.http.PlainHttpConnection.shutdown(java.httpclient@9-ea/PlainHttpConnection.java:231)
at java.net.http.PlainHttpConnection.asyncRead(java.httpclient@9-ea/PlainHttpConnection.java:252)
at java.net.http.PlainHttpConnection$ReadEvent.handle(java.httpclient@9-ea/PlainHttpConnection.java:332)
at java.net.http.HttpClientImpl$SelectorManager.handleEvent(java.httpclient@9-ea/HttpClientImpl.java:305)
at java.net.http.HttpClientImpl$SelectorManager.run(java.httpclient@9-ea/HttpClientImpl.java:275)