Fix Array#sample randomness

[takaram]

Result of Array#sample is biased as shown below.
This PR fixes it.

Test script

n = 2
array = [0, 1, 2]
h = array.permutation(n).map { [_1, 0] }.to_h

p 100000.times.map { array.sample(n) }.tally(h)

Result

n = 2

{[0, 1]=>22253, [0, 2]=>11210, [1, 0]=>22379, [1, 2]=>11014, [2, 0]=>10970, [2, 1]=>22174}

n = 3

{[0, 1, 2]=>14988, [0, 2, 1]=>18262, [1, 0, 2]=>18453, [1, 2, 0]=>18645, [2, 0, 1]=>14864, [2, 1, 0]=>14788}

Note that the implemetation employs Fisher–Yates shuffle algorithm.

[elia]

Thanks!