http2/h2demo: flush headers earlier in demo /ECHO handler
http2/h2demo: flush headers earlier in demo /ECHO handler

It confused somebody who thought things were hanging because they had
expected to see a response before they started streaming data.

Change-Id: If672956efde3756c966b0c88b9c15ed21daeccba
Reviewed-on: https://go-review.googlesource.com/125644
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/net