cmd/getgo: add a user-agent to download requests

Change-Id: Ic8fa61e53e2d06a02b06b685bd1082423df00825
Reviewed-on: https://go-review.googlesource.com/52530
Run-TryBot: Jaana Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
1 file changed