webext-signed-pages does not work with utf8 pages

[monperrus]

I love the concept of web page signing, and this implementation is super cool. I can get it working on Firefox 125.

However, verification does not work on pages with utf8 content.

Per my tests, it seems to fail on the firefox side, the signature is OK on the server side.

[MashedD]

I confirm that problem exists with UTF-8 pages. I've made a simple page with letter "ń" in header's title - yields red shield. Changed to letter "n" - green shield. Edit: even "'" (apostrophe) seems to be causing issues as well.