[MlMt] Possible bug: switching into Markdown mode saves a draft without prompting

[Sam Hathaway]

## Scenario 1: No Markdown, works as expected ##

### Steps to replicate ###

1. Reply to a text-only message.
2. Type some non-Markdown text.
3. Hit Cmd-W to close.
4. Click “Don’t Save”.

### Expected/actual result ###

* No message is added to the Drafts folder.

## Scenario 2: Markdown, draft is silently saved ##

### Steps to replicate ###

1. Reply to a text-only message.
2. Type some Markdown text, e.g. `Hello, *world!*`
3. Type some more text, e.g. `Goodbye, world!`
4. Hit Cmd-W to close.
5. Click “Don’t Save”.

### Expected result ###

* No message is added to the Drafts folder.

### Actual result ###

* There is a new message in the Drafts folder that contains the state of 
the message _at the time that it was converted to Markdown format._ In 
this example, it will contain `Hello, *world!*` but _not_ `Goodbye, 
world!`.

## Notes ##

* If you close the message right after it switches into Markdown mode, 
MailMate doesn’t prompt for whether to save the message, but the draft 
is created anyway.
* This probably happens with new messages (not replies) as well. I 
haven’t checked.
* You probably need to have Markdown turned on in the first place for 
this to happen.
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