RFR 8163561, add a test for Proxy Authentication in HTTP/2 Client API

[Felix Yang]

Hi all,

    please review a new test, which tries to verify 
"Proxy-Authenticate" header can be handled correctly.

Bug: https://bugs.openjdk.java.net/browse/JDK-8163561

Webrev: http://cr.openjdk.java.net/~xiaofeya/8163561/webrev.00/


Thanks,

Felix