testServerResponse had dead code -- it was not checking the return
value from the handler func. This fix revealed two broken tests, which
were also fixed.

Change-Id: I7cd6f6c71dbd909fa3b0aff7f98a47dade42913b
Reviewed-on: https://go-review.googlesource.com/29434Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>