SF string quoting

[bsdphk]

In the grease section:

        When adding arbitrary values to brands, user agents MUST
        make sure that receivers of the header adhere to Structured
        Header parsing, by adding escaped double-quotes, commas and
        semi-colons to those values. The purpose of this is to make
        non-compliant server implementations immediately aware that
        their parsing code is inadequate.

In SF strings only double-quote and back-slash needs to, or can be, escaped.