net/http: avoid appending an existing trailing slash to path again

net/http: avoid appending an existing trailing slash to path again

This CL is similar to CL 562557, and it takes over CL 594175.

While here, unrelatedly remove mapKeys function, use slices.Sorted(maps.Keys(ms))
to simplify code.

Fixes #67657

Change-Id: Id8b99216f87a6dcfd6d5fa61407b515324c79112
Reviewed-on: https://go-review.googlesource.com/c/go/+/594737
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Joedian Reid <joedian@google.com>