IMAP message handling produces issues with no description

Symptoms:

• Using JIRA 3.13.1
• Using IMAP service to create issues
• Issues are created, but the description is empty
• The following warning appears in your logs:

  [com.atlassian.mail.MailUtils] Unable to extract text from MIME part with Content-Type 'TEXT/PLAIN; format=flowed; charset=US-ASCII

This bug can easily be reproduced using 3.13.1 standalone and an IMAP mailbox. All mail clients appear to be affected.