[perl #127779] Inconsistency with non-int $k in “combinations” (combinations 2, 0.5)

[Christian Bartolomaeus via RT]

The implementation has changed with rakudo commit f220a5ed so that Int() is 
called on $k (and $n).

The examples work consistently now, but it looks like there are no tests for 
this in roast:

$ perl6-m -e 'say combinations 2, 0.5'
(())

$ perl6-m -e 'say combinations 2, 1.5'
((0) (1))

$ perl6-m -e 'say combinations 4, 2.5'
((0 1) (0 2) (0 3) (1 2) (1 3) (2 3))