Use case insensitive compare when validating the fever api key

User aschilling made some extensive client tests with miniflux and noticed that Mr Reader (iOS) and ReadKit (OSX) are sending the API key in upper case.

fever/index.php

@@ -46,7 +46,7 @@ function auth()
 
     $response = array(
         'api_version' => 3,
-        'auth' => (int) (@$_POST['api_key'] === $api_key),
+        'auth' => (int) (isset($_POST['api_key']) && (strcasecmp($_POST['api_key'],  $api_key) === 0 )),
         'last_refreshed_on_time' => time(),
     );