Also don't nil out the Request or Response Body on error. Just leave
it in its previous broken state. The docs now say it's undefined, but
it always was.

Fixes #14036

Change-Id: I7fe175a36cbc01b4158f4dffacd8733b2ffa9999
Reviewed-on: https://go-review.googlesource.com/18726Reviewed-by: Rob Pike <r@golang.org>