Allowing NULL disc.method in the case variables are already discrete

[DouglasMesquita]

Link to the issue

close #29

How to test the changes

data(iris)

iris$disc1 <- cut(iris$Sepal.Length, breaks = 2, include.lowest = TRUE, right = TRUE)
iris$disc2 <- cut(iris$Petal.Length, breaks = 2, include.lowest = TRUE, right = TRUE)

iris_cat <- iris[c("Species", "disc1", "disc2")]

# no disc.method
prepareTransactions(formula = Species ~ disc1 + disc2, data = iris_cat, disc.method = NULL)