Give *recordingConn the correct WriteTo signature
to be an io.WriterTo. This makes vet happy.
It also means that it'll report errors,
which were previously being ignored.

Updates #11041

Change-Id: I13f171407d63f4b62427679bff362eb74faddca5
Reviewed-on: https://go-review.googlesource.com/27121
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>