fixed a bug that $differ flag is not set.

Hi.

I encountered a bug similar to the following.

    $email->walk_parts(
        sub {
            if ($_[0]->body eq ...) {
                    $_[0] = Email::MIME->create( ... );
            }
        }
    );
    $email->as_string; # Not been changed. Original.

Find the bug, I was corrected.

It is a supplement to, but not correct test.

    60 });
    61
    62 is($called_parts_set, 1, "called parts_set once");
        # The number of times that is called from Email/Mime.pm line 227.
        # It is not the thing was called from within the function Email::Mime::walk_parts.
    63
    64 done_testing;

Please merge this If it's ok with you.

Cheers,
Tomohiro Hosaka