make HTTPClient loadable via autoloader

this fixes the HTTP tests which do test the base class directly instead
of the DokuHTTPClient subclass

inc/load.php

@@ -49,6 +49,7 @@ function load_autoload($name){
     static $classes = null;
     if(is_null($classes)) $classes = array(
         'DokuHTTPClient'        => DOKU_INC.'inc/HTTPClient.php',
+        'HTTPClient'            => DOKU_INC.'inc/HTTPClient.php',
         'JSON'                  => DOKU_INC.'inc/JSON.php',
         'adLDAP'                => DOKU_INC.'inc/adLDAP.php',
         'Diff'                  => DOKU_INC.'inc/DifferenceEngine.php',