Empty subject when entering longer then 72 chars in CiviMail.

I'm using version 3.1.3 Drupal PHP5 26406. We will loss subject when encoding a text over 72 chars(maybe 72 or higher) in subject. So user will get empty subject if we fill longer subject in CiviMail.

After 8 hours of tracing, I got the problem in "CRM_Utils_Mail_FixedMailMIME:_encodeHeaders". After the "parent::_encodeHeaders", we will get the empty subject return.

How to reproduce this issue. Please use civimail.
1. create new mailing
2. in the step 3 Mailing Content, fill in long long subject
3. in the step 4, send a test mail.
4. check the mailbox, missing subject.

After discover this, I used drupal mime header encoding function to replace old one, then this issue solved.

Drupal mime_header_encode function links below:
http://api.drupal.org/api/function/mime_header_encode

My dirty patch is in attachment.