update CONFLICT to 409

Author: satoshi-ohshige

src/StatusCode.php

@@ -56,7 +56,7 @@ final class StatusCode
     const NOT_ACCEPTABLE = 406;
     const PROXY_AUTHENTICATION_REQUIRED = 407;
     const REQUEST_TIME_OUT = 408;
-    const CONFLICT = 408;
+    const CONFLICT = 409;
     const GONE = 410;
     const LENGTH_REQUIRED = 411;
     const PRECONDITION_FAILED = 412;