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/mu.scm

@@ -38,8 +38,7 @@
     mu:header
     ;; message accessors
     mu:field:bcc
-    mu:field:body-html
-    mu:field:body-txt
+    mu:field:body
     mu:field:cc
     mu:field:date
     mu:field:flags
@@ -54,6 +53,9 @@
     mu:field:tags
     mu:field:timestamp
     mu:field:to
+    ;; deprecated message accessors
+    mu:body-html
+    mu:body-txt
     ;; contact funcs
     mu:name
     mu:email
@@ -125,8 +127,7 @@
 	(export method-name)))))
 
 (define-getter mu:bcc	     mu:field:bcc)
-(define-getter mu:body-html  mu:field:body-html)
-(define-getter mu:body-txt   mu:field:body-txt)
+(define-getter mu:body	     mu:field:body)
 (define-getter mu:cc	     mu:field:cc)
 (define-getter mu:date	     mu:field:date)
 (define-getter mu:flags	     mu:field:flags)
@@ -142,6 +143,16 @@
 (define-getter mu:timestamp  mu:field:timestamp)
 (define-getter mu:to	     mu:field:to)
 
+(define-method (mu:body-html (msg <mu:message>))
+  "The HTML body isn't stored separately anymore, so this method can't return
+anything useful. We keep it for backwards compatibility."
+  #f)
+
+(define-method (mu:body-txt (msg <mu:message>))
+  "The text body isn't stored separately anymore. This method is now a synonym
+for mu:body."
+  (mu:body msg))
+
 (define-method (mu:header (msg <mu:message>) (hdr <string>))
   "Get an arbitrary header HDR from message MSG; return #f if it does
 not exist."