soup_header_g_string_append_param: if @value is a token, don't quote it

soup_header_g_string_append_param: if @value is a token, don't quote it

also add soup_header_g_string_append_param_quoted(), for places that
need to guarantee they're adding a quoted-string, not a token, even if
a token would be syntactically correct. (eg, digest auth, where lots of
implementations are very very picky)