Wrong character is produced when indent contains full-width (zenkaku) space

[makenowjust]

Describe the bug

Wrong character is produced by heredoc.Doc function when indent contains full-width (zenkaku, u+3000) space.

To Reproduce

With:

package main

import (
	"fmt"

	"github.com/MakeNowJust/heredoc"
)

func main() {
	fmt.Printf("%#+v\n", heredoc.Doc("\n\u3000hello"))
}

Shows this result: "\x80\x80hello". "\x80" is wrong character.

Expected behavior

"hello" is correct result.

[makenowjust]

This bugs comes from using unicode.IsSpace, and Go's string slice counts its size as bytes, not as runes.

Package heredoc is used for writing raw string as heredoc-like, so it should follow Go language spec with respect to space character.

Go language spec says these characters are treated as space:

• spaces U+0020
• horizontal tabs U+0009
• carriage returns U+000D
• newlines U+000A

Newlines and carriage returns are known as newline character. As a conclusion, heredoc should treat only spaces and horizontal tabs as indent characters.