net/http: refine trailing-slash redirect logic

net/http: refine trailing-slash redirect logic

Do not add a trailing slash and redirect if the path already
ends in a slash.

Also, and unrelatedly, add a test for cleanPath.

Fixes #65624.

Change-Id: Ifcf9edc929d2eb6db88132c09d2bade85c5dda3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/562557
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>