x/text: ianaindex returns no encoding for gb2312

[crawshaw]

Using the golang.org/x/text/encoding/ianaindex package.

        encoding, err := ianaindex.MIME.Encoding("gb2312")
        fmt.Println(encoding, err) // prints <nil> <nil>

I believe it should be returning simplifiedchinese.HZBG2312.

cc @mpvl

[meirf]

This specific behavior has been around since Encoding was added.

To fix this particular example, would probably just need an entry:
enc2025: simplifiedchinese.HZGB2312,
Though I wonder if there's a way to generate the encodings from a public source.

The code has this:

// TODO: the coverage of this index is rather spotty. Allowing users to set
// encodings would allow:
// - users to increase coverage
// - allow a partially loaded set of encodings in case the user doesn't need to
//   them all.
// - write an OS-specific wrapper for supported encodings and set them.