Mu4e: Ignore case when matching mu4e context

This allows users to have different casing in their mu4e context names
and the corresponding directories. Since mu4e's context switching
automatically selects the letter from the context name, this allows
users to set two contexts starting with the same letter to uppercase and
lowercase variants, while leaving the directories lowercase.

modules/email/mu4e/autoload/email.el

@@ -36,7 +36,7 @@ default/fallback account."
                     (lambda (msg)
                       (when msg
                         (string-prefix-p (format "/%s" label)
-                                         (mu4e-message-field msg :maildir))))
+                                         (mu4e-message-field msg :maildir) t)))
                     :vars letvars)))
       (add-to-list 'mu4e-contexts context (not default-p))
       context)))