Update openid-connect-generic-client-wrapper.php

Add id_token_hint to end session logout redirect url 
http://openid.net/specs/openid-connect-session-1_0.html#RPLogout

includes/openid-connect-generic-client-wrapper.php

@@ -260,8 +260,11 @@ class OpenID_Connect_Generic_Client_Wrapper {
 			$redirect_url = home_url( $redirect_url );
 		}
 
-		$url .= 'post_logout_redirect_uri=' . urlencode( $redirect_url );
+		$user_id = wp_get_current_user()->ID;
-		return $url;
+                $token_response = get_user_meta( $user_id, 'openid-connect-generic-last-token-response', true );
+                $id_token_hint = $token_response['id_token'];
+                $url .= 'id_token_hint='.$id_token_hint.'&post_logout_redirect_uri=' . urlencode( $redirect_url );
+                return $url;
 	}
 
 	/**