This change modifies URL.String to prepend "./" to a relative URL which
contains a colon in the first path segment.

Per RFC 3986 §4.2:

> A path segment that contains a colon character (e.g., "this:that")
> cannot be used as the first segment of a relative-path reference, as
> it would be mistaken for a scheme name.  Such a segment must be
> preceded by a dot-segment (e.g., "./this:that") to make a relative-
> path reference.

https://go-review.googlesource.com/27440 corrects the behavior for http.FileServer,
but URL.String will still return an invalid URL. This CL reverts the changes to
http.FileServer as they are unnecessary with this fix.

Fixes #17184

Change-Id: I9211ae20f82c91b785d1b079b2cd766487d94225
Reviewed-on: https://go-review.googlesource.com/29610Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>