guile: Fix the mu:body message method

Somewhere along the way, the body-txt and body-html fields were merged
into a single body field. Later, commit 8eac392221 ("guile: re-enable
the guile support") finally removed support for Field::Id::BodyHtml from
mu:c:get-field.

Unfortunately mu.scm and the documentation are still stuck in the past, so
update them. mu:body-txt is now a synonym for mu:body, and mu:body-html
always returns #f.

I wanted to add a mu:body test also for the rfc822.1 message, but there's
currently a bug where its body text is is duplicated (issue #2802), so the
test would fail.

guile/tests/test-mu-guile.scm

@@ -84,6 +84,22 @@ exec guile -e main -s $0 $@
     (str-equal-or-exit (mu:to      msg) "Democritus <demo@example.com>")
     (str-equal-or-exit (mu:from    msg) "Richard P. Feynman <rpf@example.com>")
     ;;(str-equal-or-exit (mu:header msg "Content-Transfer-Encoding") "8bit")
+    (str-equal-or-exit (mu:body msg)
+                       (string-join
+                        '("If, in some cataclysm, all scientific knowledge were to be destroyed,"
+                          "and only one sentence passed on to the next generation of creatures,"
+                          "what statement would contain the most information in the fewest words?"
+                          "I believe it is the atomic hypothesis (or atomic fact, or whatever you"
+                          "wish to call it) that all things are made of atoms — little particles"
+                          "that move around in perpetual motion, attracting each other when they"
+                          "are a little distance apart, but repelling upon being squeezed into"
+                          "one another. In that one sentence you will see an enormous amount of"
+                          "information about the world, if just a little imagination and thinking"
+                          "are applied.\n") "\n"))
+    (str-equal-or-exit (mu:body-txt msg) (mu:body msg))
+    (let ((got (mu:body-html msg)))
+      (if got
+          (error-exit "Expected #f, got ~a" got)))
 
     (if (not (equal? (mu:priority msg) mu:prio:high))
       (error-exit "Expected ~a, got ~a"  (mu:priority msg) mu:prio:high))))