Heading attribute panics

[bep]

The source below is taken from the README.

package main

import (
	"bytes"

	"github.com/yuin/goldmark"
	"github.com/yuin/goldmark/parser"
)

func main() {
	md := goldmark.New(
		goldmark.WithParserOptions(
			parser.WithAttribute(),
		),
	)
	source := []byte(`
## heading {#id .className attrName=attrValue class="class1 class2"}
`)
	var buf bytes.Buffer
	if err := md.Convert(source, &buf); err != nil {
		panic(err)
	}
}

Panics:

panic: interface conversion: interface {} is [][]uint8, not []uint8

goroutine 1 [running]:
github.com/yuin/goldmark/renderer/html.(*Renderer).RenderAttributes(0xc00009cae0, 0x12262c0, 0xc000119d80, 0x1227860, 0xc0001b2000)
	/Users/bep/go/pkg/mod/github.com/yuin/[email protected]/renderer/html/html.go:513 +0x227
github.com/yuin/goldmark/renderer/html.(*Renderer).renderHeading(0xc00009cae0, 0x12262c0, 0xc000119d80, 0xc000184140, 0x49, 0x49, 0x1227860, 0xc0001b2000, 0x2001, 0x8, ...)
	/Users/bep/go/pkg/mod/github.com/yuin/[email protected]/renderer/html/html.go:208 +0x11f
github.com/yuin/goldmark/renderer.(*renderer).Render.func2(0x1227860, 0xc0001b2000, 0x1, 0x0, 0x12262c0, 0xc000119d80)
	/Users/bep/go/pkg/mod/github.com/yuin/[email protected]/renderer/renderer.go:167 +0x108
github.com/yuin/goldmark/ast.Walk(0x1227860, 0xc0001b2000, 0xc000175e30, 0x3, 0x0)
	/Users/bep/go/pkg/mod/github.com/yuin/[email protected]/ast/ast.go:433 +0x43
github.com/yuin/goldmark/ast.Walk(0x12273e0, 0xc00011c780, 0xc000175e30, 0xc0001b8000, 0x0)
	/Users/bep/go/pkg/mod/github.com/yuin/[email protected]/ast/ast.go:439 +0x149
github.com/yuin/goldmark/renderer.(*renderer).Render(0xc0001245f0, 0x12243c0, 0xc0000909f0, 0xc000184140, 0x49, 0x49, 0x12273e0, 0xc00011c780, 0xc000124501, 0xc0000909f0)
	/Users/bep/go/pkg/mod/github.com/yuin/[email protected]/renderer/renderer.go:162 +0x13c
github.com/yuin/goldmark.(*markdown).Convert(0xc000119a00, 0xc000184140, 0x49, 0x49, 0x12243c0, 0xc0000909f0, 0x0, 0x0, 0x0, 0xc000064058, ...)
	/Users/bep/go/pkg/mod/github.com/yuin/[email protected]/markdown.go:117 +0xe3

[yuin]

Thanks for reporting this. This bug was introduced in 667a292 that changes the attribute parsing strategy. I've fixed the issue and made a new release.