Added comment verbiage to 306 status

- No longer defined authoritatively.

Author: weierophinney

src/Response.php

@@ -52,7 +52,7 @@ class Response implements ResponseInterface
         303 => 'See Other',
         304 => 'Not Modified',
         305 => 'Use Proxy',
-        306 => 'Switch Proxy', // Deprecated
+        306 => 'Switch Proxy', // Deprecated to 306 => '(Unused)'
         307 => 'Temporary Redirect',
         308 => 'Permanent Redirect',
         // CLIENT ERROR