message::prepare_payload replaces message::prepare (API Change):

Actions Required:

* Change calls to msg.prepare to msg.prepare_payload. For messages
  with a user-defined Fields, provide the function prepare_payload_impl
  in the fields type according to the Fields requirements.

test/http/doc_snippets.cpp

@@ -49,7 +49,7 @@ void fxx() {
     res.result(status::ok);
     res.set(field::server, "Beast");
     res.body = "Hello, world!";
-    res.prepare();
+    res.prepare_payload();
 
 //]
 }