Default email MIME type is not valid

See CONF-3333

"First, how the PreRenderedMailNotificationQueueItems get to the error queue. Most of our users did not bother choosing between text and html email notifications, so their confluence.prefs.email.mimetype comes from preferences-default.xml and is equal to "html". Due to apparent typo in MailNotificationQueueItem#getMimeTypeForUser (MailNotificationQueueItem.java around line 159), string "html" is not considered to represent html content and is used as is by SMTPMailServerImpl. Since "html" is not know mime type, send message fails with javax.mail.ParseException. By the way, ErrorQueuedTaskQueue#handleException looses nested exceptions and stack trace and all I saw in the logs was almost useless "send message failed"."