Allow digits! to populate a SubArray from a pre allocated Array

Issue #7948 added digits! for Arrays. I wanted to use it to fill a pre allocated 2x4 array so:
let a = zeros(Int,(2,4))
    for i in 0:3
        digits!(sub(a,:,i+1),i,2)
    end
    println(a)
end

Therefore I made the type of digits! parameter AbstractArray instead of Array.