LP#1801163: (follow-up) deal with header fields that contain Unicode strings

LP#1801163: (follow-up) deal with header fields that contain Unicode strings

Since Email::MIME->header_str_set() expects Unicode strings, not octets,
use decode_utf8() on the header values to

To test
-------
[1] Update the biblio.format.record_entry.print A/T event definition
    to include non-ASCII characters in the subject and from headers.
[2] Upon applying the patch series, verifying that the strings provided
    in step 1 are not mangled when the email is sent.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>