LibWeb: Make the WPT encoding/legacy-mb-* tests pass

[awesomekling]

These tests make up a huge part of the WPT score, so it would be nice to get them out of the way.

I'm talking specifically about:

• encoding/legacy-mb-japanese
• encoding/legacy-mb-korean
• encoding/legacy-mb-schinese
• encoding/legacy-mb-tchinese

From my understanding, we need to implement form submission with encodings other than UTF-8.

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[tcl3]

Fixed by #975

[awesomekling]

Thank you @BenJilks for contributing to close this issue! ⭐

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