add http status code 201 to show the right status message in return headers (#402)

httplib.h

@@ -1599,6 +1599,7 @@ inline const char *status_message(int status) {
   switch (status) {
   case 100: return "Continue";
   case 200: return "OK";
+  case 201: return "Created";
   case 202: return "Accepted";
   case 204: return "No Content";
   case 206: return "Partial Content";