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.

example/server-framework/http_base.hpp

@@ -50,7 +50,7 @@ protected:
         res.set(beast::http::field::server, server_name_);
         res.set(beast::http::field::content_type, "text/html");
         res.body = "Bad request";
-        res.prepare();
+        res.prepare_payload();
         return res;
     }