Fix bug#10085, decoding of mail headers with ASCII-incompatible encodings

When the postmaster reads data from an email header, it tries to decode it with the same character encoding as is specified for the body.

While this is technically wrong, it works fine for encodings that are a superset of ASCII, it fails pretty badly for UTF-7 and other encodings that are not a superset of ASCII.

This pull request makes postmaster only decode if the encoding is a superset of ASCII, falling back to ASCII otherwise.