Messed up parsing for X-Matrix Authorization header

The parsing of X-Matrix Authorization headers here is highly dubious and will be wrong for maliciously crafted inputs with silly quoting/escaping.

The format is described in RFC7235 using the standard quoted-string from RFC7230, yet I've not found any library functions in http to deal with those.

The creation of X-Matrix Authorization headers here is similarly dubious and will escape weird key IDs and signatures incorrectly, but since we just generated them immediately prior, that shouldn't be an issue.