`ispell-message' don't spell the subject line if it starts with a square bracket

[Emanuel Berg]

Start with -Q, then do `compose-mail', write something which
is incorrectly spelled in the Subject header line, then do
`ispell-message'.

It works but if you start the subject line with a left square
bracket, [, spelling the subject line doesn't happen. If you
remove it, the subject line gets spell-checked as well as
the body.

You cannot prevent the body from being spelled with
this method.

Probably a bug, right?

So to exemplify how it looks to me

To: 
Subject: [this will prevent spelling] spel1 th1s
From: Emanuel Berg <incal@ebc.mail-host-address-is-not-set>
--text follows this line--
spel1 th1s

--
hello

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