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/examples/org2mu4e

@@ -43,7 +43,7 @@ exec guile -e main -s $0 $@
     (org-mu4e-link msg)
     (if tag (string-concatenate `(":" ,tag "::")) "")
     (or (mu:from msg) "?")
-    (let ((body (mu:body-txt msg)))
+    (let ((body (mu:body msg)))
       (if (not body) ;; get a 'summary' of the body text
 	"<no plain-text body>"
 	(string-map

guile/mu-guile.texi

@@ -391,8 +391,7 @@ properties, please refer to the @t{mu-find} man-page.
 
 @itemize
 @item @code{(mu:bcc msg)}: the @t{Bcc} field of the message, or @t{#f} if there is none
-@item @code{(mu:body-html msg)}: : the html body of the message, or @t{#f} if there is none
-@item @code{(mu:body-txt msg)}: the plain-text body of the message, or @t{#f} if there is none
+@item @code{(mu:body msg)}: the body of the message, or @t{#f} if there is none
 @item @code{(mu:cc msg)}: the @t{Bcc} field of the message, or @t{#f} if there is none
 @item @code{(mu:date msg)}: the @t{Date} field of the message, or 0 if there is none
 @item @code{(mu:flags msg)}: list of message-flags for this message

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."

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))))