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/string_view_body.cpp

@@ -31,7 +31,7 @@ public:
         req.version = 11;
         req.method(verb::post);
         req.target("/");
-        req.prepare();
+        req.prepare_payload();
         static_buffer_n<512> b;
         ostream(b) << req;
         string_view const s{