Found when porting encoding WPT into core.

Spec: https://encoding.spec.whatwg.org/#concept-td-serialize

If encoding is UTF-8, UTF-16BE, or UTF-16LE, and ignore BOM flag and BOM seen flag are unset, then:

• If token is U+FEFF, then set BOM seen flag.
• Otherwise, if token is not end-of-stream, then set BOM seen flag and append token to output.
• Otherwise, return output.

Failing tests:

• wpt/encoding/textdecoder-byte-order-marks.any.js
• wpt/encoding/textdecoder-copy.any.js