add test for invariable special tokens index in dictionary
add test for invariable special tokens index in dictionary

jsenellart/OpenNMT,jungikim/OpenNMT,OpenNMT/OpenNMT,monsieurzhang/OpenNMT,da03/OpenNMT,jsenellart/OpenNMT,jsenellart-systran/OpenNMT,jsenellart/OpenNMT,da03/OpenNMT,jsenellart-systran/OpenNMT,monsieurzhang/OpenNMT,OpenNMT/OpenNMT,jungikim/OpenNMT,jungikim/OpenNMT,monsieurzhang/OpenNMT,jsenellart-systran/OpenNMT,da03/OpenNMT,OpenNMT/OpenNMT,cservan/OpenNMT_scores_0.2.0,srush/OpenNMT