Request for additional findnz() method

[btmit]

I often find the following pattern useful for sparse matrices.

a = sprand(4, 4, 0.5)
b = [nnz(c) for c in eachcol(a)]

However, if I try the same thing with the findnz() it appears that this method does not exist.

b = [findnz(c)[1] for c in eachcol(a)]

ERROR: MethodError: no method matching findnz(::SubArray{Float64, 1, SparseMatrixCSC{Float64, Int64}, Tuple{Base.Slice{Base.OneTo{Int64}}, Int64}, false})

Does adding this method make sense for a future version SparseArrays?

[ViralBShah]

Yes, hopefully this should be a small amount of code to make findnz accept subarrays. If you are up for it, can you submit a PR?