http: Client test for streaming responses (no code changes)

I had a report that this was broken. It seems fine.

I think the reporter was just never flushing their response
headers.  If I omit the test server's initial Flush I get the
same behavior as reported. (a hang at Client.Get)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4552062
1 file changed