Replace sample odds ratio by unconditional MLE

The sample odds ratio is an biased estimate if it is close to zero. Use
therefore the unconditional maximum likelihood estimate like in R (see,
e.g. p. 135 in [1]  or p. 520 in [2])

[1] Anderson, Sharon Roe, et al. Statistical methods for comparative
studies: techniques for bias reduction. Vol. 170. John Wiley & Sons,
2009
[2] Hollander, Myles, Douglas A. Wolfe, and Eric Chicken. Nonparametric
statistical methods. Vol. 751. John Wiley & Sons, 2013.