net/url: prefix relative paths containing ":" in the first segment with "./"
This change modifies URL.String to prepend "./" to a relative URL which
contains a colon in the first path segment.
Per RFC 3986 ยง4.2:
> 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.