[11u] RFR: JDK-8236859 backport: WebSocket over authenticating proxy fails with NPE

[Ilarion Nakonechnyy]

Dear sirs, I’d like to backport JDK-8236859 to JDK11.

The bug causes jtreg test java/net/httpclient/websocket/WebSocketProxyTest.java to fail with timeout.
Was caught on x86_64 platform, but seems that it’s platform independent.

Patch applied unclear, manually merged files src/java.net.http/share/classes/jdk/internal/net/http/AuthenticationFilter.java and src/java.net.http/share/classes/jdk/internal/net/http/HttpResponseImpl.java
Jdk was built on Ubuntu x86_64 and tested with tier1

Could you please review my changes?

Bug: <https://bugs.openjdk.java.net/browse/JDK-8260378>  https://bugs.openjdk.java.net/browse/JDK-8236859
Webrev: <https://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8260378/jdk11/02/webrev/>  http://cr.openjdk.java.net/~yan/8236859/webrev.11.0/

Original change: https://hg.openjdk.java.net/jdk/jdk/rev/ed8e7bf32188