Issue about BLEU smoothing method 4

[OpenSource-fan]

NLTK3.5 implements smoothing method 4 proposed in the paper [https://www.aclweb.org/anthology/W14-3346/ ].
However, we suspect that the implementation is incorrect. The code at

nltk/nltk/translate/bleu_score.py

Lines 582 to 585 in d0f54c2

	incvnt = i + 1 * self.k / math.log(
	hyp_len
	) # Note that this K is different from the K from NIST.
	p_n[i] = incvnt / p_i.denominator

is problematic.
This leads to the p_n[i] greater than 100% in some case. When hyp_len < 4 in 4-gram, p_n[i] can be assigned with a percentage number that is much greater than 100% (or even > 500%).
In fact, the p_n[i] should not be greater than 100%.
The correct implementation is p_n[i] = (1 / incvnt) / p_i.denominator in line585.

[stevenbird]

Thanks @anonymityBoy. Would you please submit a pull request?

[stevenbird]

Thanks @anonymityBoy... can I trouble you to add a regression test or two here please?

[OpenSource-fan]

Sure. I have added.