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.

include/beast/websocket/impl/stream.ipp

@@ -218,7 +218,7 @@ build_response(http::header<true,
             res.version = req.version;
             res.result(http::status::bad_request);
             res.body = text;
-            res.prepare();
+            res.prepare_payload();
             decorate(res);
             return res;
         };
@@ -248,7 +248,7 @@ build_response(http::header<true,
             res.result(http::status::upgrade_required);
             res.version = req.version;
             res.set(http::field::sec_websocket_version, "13");
-            res.prepare();
+            res.prepare_payload();
             decorate(res);
             return res;
         }