x/text/secure/bidirule: label starting with EN UTF8 property should be invalid

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (`go version`)?

go version go1.9 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (`go env`)?

GOARCH="amd64" GOBIN="" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOOS="darwin" GOPATH="/Users/mgalkowski/work/go" GORACE="" GOROOT="/usr/local/Cellar/go/1.9/libexec" GOTOOLDIR="/usr/local/Cellar/go/1.9/libexec/pkg/tool/darwin_amd64" GCCGO="gccgo" CC="clang" GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/cs/93b4_xbx4l5gn1wx0f69p3d40000gn/T/go-build033566806=/tmp/go-build -gno-record-gcc-switches -fno-common" CXX="clang++" CGO_ENABLED="1" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config"

What did you do?

If possible, provide a recipe for reproducing the error. A complete runnable program is good.

package main

import (
    "fmt"

    "golang.org/x/text/secure/bidirule"
)

func main() {
    var invalidLabel = "0aaaa"
    var invalidLabelEscaped = "\u0030aaaaa"

    fmt.Println(bidirule.ValidString(invalidLabel))
    fmt.Println(bidirule.ValidString(invalidLabelEscaped))
}

What did you expect to see?

false
false

What did you see instead?

true 
true

According to RFC 5893:

The first character must be a character with Bidi property L, R,
       or AL.  If it has the R or AL property, it is an RTL label; if it
       has the L property, it is an LTR label.

0 (\u0030) have EN (european number) property, to it is not allowed at the beginning of the label. This package is not conformant with the BIDI RFC.

golang / go