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/http-client-ssl/http_client_ssl.cpp

@@ -64,7 +64,7 @@ int main()
     req.set(http::field::host, host + ":" +
         boost::lexical_cast<std::string>(sock.remote_endpoint().port()));
     req.set(http::field::user_agent, "Beast");
-    req.prepare();
+    req.prepare_payload();
 
     // Write the HTTP request to the remote host
     http::write(stream, req, ec);