fix #111: shouldRetainParam() should handle 'code'

shouldRetainParam function doesn't work as advertised in a comment,
return false if given 'code', 'code=', or 'code=a'

Author: Davies Liu

src/base_facebook.php

@@ -1258,7 +1258,8 @@ protected function getCurrentUrl() {
    */
   protected function shouldRetainParam($param) {
     foreach (self::$DROP_QUERY_PARAMS as $drop_query_param) {
-      if (strpos($param, $drop_query_param.'=') === 0) {
+      if ($param === $drop_query_param ||
+          strpos($param, $drop_query_param.'=') === 0) {
         return false;
       }
     }