title: Wordpress Rest API returns error
body: <p>I'am developing a plugin for wordpress and have trouble with the Rest API.
  On my test server it works without a problem. (v4.6.6)
  On a different server (v4.4.10) the API returns this error message:</p>
  
  <pre><code>{"code":"rest_invalid_handler","message":"
  Der Handler f\u00fcr die Route ist ung\u00fcltig","data":{"status":500}}%
  </code></pre>
  
  <p>The message is in german and means "The handler for the route is invalid." Don't understand why they translate the error messages for an API. Makes no sense for me. :)</p>
  
  <p>The routes on the <a href="http://domain/wp-json" rel="noreferrer">http://domain/wp-json</a> are equal.
  Maybe an problem with the different WP versions?</p>
  
  <p>Definition of the route:</p>
  
  <pre><code>function __construct() {
      add_action( 'rest_api_init', function(){
          register_rest_route( 'test_namespace', 'ping', array(
              'methods' =&gt; 'POST',
              'callback' =&gt; array($this, 'ping_test'),
              'permission_callback' =&gt;  array($this, 'myhacks_permission_callback'),
          ) );
      } );
  }
  </code></pre>
  
  <p>Thanks for help.</p>