Fix external format for message headers

mailfilter-parse.lisp

     (cl-ppcre:scan regex string)))
 
 (defun message-match (message pattern)
-  (let ((test-func (make-message-regex-test (gethash "regex" pattern)))
-        (stream (mel:message-header-stream message))
-        (test))
+  (let* ((test-func (make-message-regex-test (gethash "regex" pattern)))
+         (sb-impl::*default-external-format* :latin-1)
+         (stream (mel:message-header-stream message))
+         (test))
     (loop for line = (read-line stream nil 'eof) do
       (or (eq line 'eof) (setf test (funcall test-func line)))
           until (or (eq line 'eof) test))