Check the authorization status is "valid" rather than "ready"

According to https://tools.ietf.org/html/rfc8555#section-7.1.4 the
status of an authorization object can only be "pending", "valid", or
"revoked".

An order can have a status of "ready" but never an authorization.

src/Client.php

@@ -286,7 +286,7 @@ class Client
             sleep(1);
         } while ($maxAttempts < 0 && $data['status'] == 'pending');
 
-        return (isset($data['status']) && $data['status'] == 'ready');
+        return (isset($data['status']) && $data['status'] == 'valid');
     }
 
     /**