sieve-mode: support "body" test command

* lisp/gnus/sieve-mode.el (sieve-font-lock-keywords):
Add missing "body" test command.

lisp/gnus/sieve-mode.el

@@ -140,7 +140,8 @@
 	   'sieve-action-commands-face)
      ;; test commands
      (cons (regexp-opt '("address" "allof" "anyof" "exists" "false"
-			 "true" "header" "not" "size" "envelope")
+			 "true" "header" "not" "size" "envelope"
+                         "body")
                        'words)
 	   'sieve-test-commands-face)
      (cons "\\Sw+:\\sw+"