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/doc/http_examples.hpp

@@ -320,7 +320,7 @@ void do_server_head(
         // set of headers that would be sent for a GET request,
         // including the Content-Length, except for the body.
         res.result(status::ok);
-        res.content_length(payload.size());
+        res.set(field::content_length, payload.size());
 
         // For GET requests, we include the body
         if(req.method() == verb::get)