fix(3p/lisp/mime4cl): make MIME-BODY-STREAM always return characters

Because OPEN-DECODED-FILE-PORTION only knows about transfer encodings it
would only return a character stream for 7bit encoded bodies. This
causes inconsistent behavior where some bodies would return binary and
some character streams. To fix this, we specialize MIME-BODY-STREAM for
MIME-TEXT parts which may or may not be a good enough solution.

We may actually want to make MIME-BODY-STREAM binary always and let the
user handle decoding?! This may be a good idea to take care after yet
another stream machinery redesign.

Since the mime4cl test suite doesn't test MIME-BODY-STREAM (much), add a
message generated by notemap that hits this issue to the mblog golden
test suite.

Change-Id: Ie340c42ced6c693af9b3c84b177408d6b6d2c9c4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12913
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

third_party/lisp/mime4cl/endec.lisp

@@ -656,6 +656,7 @@ sequence, a charset string indicating the original coding."
              (cons (flexi-streams:octets-to-string
                     (car part)
                     :external-format (flexi-streams:make-external-format
+                                      ;; TODO(sterni): sanitize charset before interning
                                       (intern (string-upcase (cdr part)) 'keyword))))
              (string part))))
     (apply #'concatenate