Added MESSAGE-ID til header array.

doc/message_array.html

@@ -18,6 +18,7 @@ $message     ["HEADER"]                    (array)
                 ["CHARSET"]                (string)
                 ["MIME"]                   (boolean)
                 ["ENCODING"]               (string)
+                ["MESSAGE-ID"]             (string)
 
              ["ENTITIES"]                  (array)
                 [0]                        (array)