net/url: preserve leading slashes when resolving path

net/url: preserve leading slashes when resolving path

When doing resolvePath, if there are multiple leading slashes in the
target, preserve them. This prevents an issue where the Go http.Client
cleans up multiple leading slashes in the Location header in a
redirect, resulting in a redirection to the incorrect target.

Fixes #21158.

Change-Id: I6a21ea61ca3bc7033f3c8a6ccc21ecaa3e996fa8
Reviewed-on: https://go-review.googlesource.com/51050
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>