[FIX] core: correctly decode 8bit mail headers
[FIX] core: correctly decode 8bit mail headers

When an header contains surrogates, an Header object is returned when
read headers from Message objects.

Also, correctly join header parts in python3.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo