parse_accept_header() split on every ',' and then on every ';', with no
idea that a parameter value can be a quoted-string (RFC 9110 §5.6.6). A
value such as text/html;profile="a,b" was cut in the middle, and the
request was answered with 400.
The parser never looks at a parameter value, so remove each
quoted-string (§5.6.4) before parsing rather than teach every split
about quotes. A header without a '"' takes the same path as before.
A quoted-string is removed wherever it appears, so a malformed value
can now be read without it instead of being rejected, and an
unterminated quote hides the rest of the value.