[Dovecot] Mailbox written with normalized headers

Over on the SpamAssassin list someone noted that some spam generators add leading spaces to the subject line, but it was impossible to detect this when using Dovecot for POP3 service. Some quick testing shows that dovecot-0.99.13-4.FC2 writes the mailbox back with the extra spaces in the header squeezed out. I think Dovecot should avoid reformatting headers any more than necessary to avoid interfering with spam detectors.