Ideas for an RFC 6532 API
Step 1:
- change the Header class in header.py to make the default charset choosable
create new classes in the headerregistry.py (e.g. UnicodeAddressHeader as inheritor of AddressHeader
- provide an alternative header map (e.g. _unicode_header_map) using the new classes
HeaderRegistry: either create an inherited class with the _unicode_header_map as default.