idna: do not check ace prefix slashes in case of acePrefix not present

[moredure]

if this rule is not deleted valid domains failed to make ascii for example http://0-------------------------------------------------------------5.com

[gopherbot]

This PR (HEAD: a3aec84) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/net/+/380514 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

This PR (HEAD: 6631f2f) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/net/+/380514 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

This PR (HEAD: 51dc05a) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/net/+/380514 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[moredure]

whatwg/url#53 (comment)