RFR: 8158651: ConcurrentModification exceptions in httpclient

[Michael McMahon]

Could I get the following small change reviewed please?

http://cr.openjdk.java.net/~michaelm/8158651/webrev.1/

The exception has been seen in some tests failing intermittently.
So, I have not added a new test

The failure occurs in Http2Connection but a similar case
was found in Stream.java

Thanks
Michael

PS I will get back to API review comments next week.