soup-multipart: Fix generation of Content-Disposition for broken servers

soup-multipart: Fix generation of Content-Disposition for broken servers

Some servers require that Content-Disposition headers use
quoted-string values, even if the provided filename can be represented
as a token. So use soup_header_g_string_append_param_quoted(), but also
change that to fall back to using RFC 5987 encoding if necessary, so
that we can still specify UTF-8-encoded filenames.