RFR 8193365: Improve interoperability between HTTP Client's BodyPublisher/BodySubscriber and Flow.Subscriber/Publisher

[Chris Hegarty]

This review is for 8193365 [1], to improve interoperability
between HTTP Client's BodyPublisher/BodySubscriber and
Flow.Subscriber/Publisher, that was raised as part of
feedback on JEP 321 [2].

http://cr.openjdk.java.net/~chegar/8193365/webrev.01/

-Chris.

[1] https://bugs.openjdk.java.net/browse/JDK-8193365
[2] 
http://mail.openjdk.java.net/pipermail/net-dev/2017-December/thread.html#11063