[BUG] M::M::Convert::MimeEntity drops content-type

Importing from a multi-part MIME::Entity object fails to take notice
of content-disposition and content-type headers. This is a shame, as
they're quite important.

    use Mail::Message::Convert::MimeEntity;
    my $convert = Mail::Message::Convert::MimeEntity->new;

    $top = MIME::Entity->build(From    => 'me@xxxxxxxxxx',
                               To      => 'you@xxxxxxxxxxxx',
                               Subject => "Hello, nurse!",
                               Data    => "This is a test");

    $top->attach(Filename => "foo.tex",
                Type     => "application/tex",
                Encoding => "base64",
                Data => "\\relax"
                );

    my $foo = $convert->from($top);
    print $foo->head.$foo->body;
    print "=== for comparison ===\n";
    $top->print(\*STDOUT);

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