One too many lines was deleted, and it would print a header multiple times.
Add a test.

Change-Id: I4906b454dbb66193d515ffacf43849ffdc2dede6
Reviewed-on: https://go-review.googlesource.com/c/142937Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>