Serializing of MIME types without space before parameters might be problematic

[annevk]

It's a little bit unclear yet, but in the context of XMLHttpRequest where implementations currently modify in-place rather than parse-then-serialize for the Content-Type header during requests, Firefox noticed some issues switching strategies:

1. https://bugzilla.mozilla.org/show_bug.cgi?id=1496754
2. https://bugzilla.mozilla.org/show_bug.cgi?id=1494443

This might end up affecting MIME type serialization slightly or end up being a change to XMLHttpRequest. I'm raising this here since I know other implementers are also looking at these algorithms and I'd like to keep everyone informed somehow.

cc @rwlbuis @MattMenke2

[annevk]

So I think I'd prefer fixing this in XMLHttpRequest and here. Make XMLHttpRequest override less Content-Type headers and serialize MIME types with the space after ;. I hope that this change isn't too problematic to adopt.

I'll prepare PRs.

cc @hsivonen @wisniewskit @domenic

[annevk]

Per https://bugzilla.mozilla.org/show_bug.cgi?id=1454325#c15 this also breaks an outlook.com service, which would also be fixed by not meddling with things if the charset is already UTF-8.

[annevk]

Closing this as whatwg/xhr#224 got merged. No changes to MIME Sniffing needed after all.