Make hz-gb-2312 appear as replacement for WHATWG.
Make hz-gb-2312 appear as replacement for WHATWG.

Fixes #84.

Keep the current HZ implementation, but surface hz-gb-2312 as
replacement in encoding_from_whatwg_label to match the spec.

This maintains WHATWG compatibility and also allows use of the actual HZ
encoding/decoding implementation.

nox/rust-encoding,lifthrasiir/rust-encoding,tempbottle/rust-encoding,tempbottle/rust-encoding,tempbottle/rust-encoding,nox/rust-encoding,lifthrasiir/rust-encoding,nox/rust-encoding,lifthrasiir/rust-encoding