gddo-server: handle trailing slash in godoc.org redirect

Fixes golang/go#43979

Change-Id: I3d02cdbfbf9ad7e76ae143b2d78aa9a7dde2097d
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/287794
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>