[Dovecot] Mailbox written with normalized headers

[Kenneth Porter]

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.