the return of AttackedText.word_diff_ratio is 1 all the time

[nohuma]

Describe the bug
the return of Attacked_text.word_diff_ratio is 1 all the time

To Reproduce
Steps to reproduce the behavior:

1. Run following command textattack ...
2. Run following code ...
3. top_member.attacked_text.words_diff_ratio(initial_result.attacked_text)
4. See error

reason
the type of AttackedText.words is list, when it is use as as input parameter of np.sum() it returns 1or 0 all the time, because it is always consider the first index of the input list