SparseMatrixCSC type conversion (sometimes) drops structural nonzeros

[sjdaines]

Update: another surprising consequence of #190 that led to hours of debugging...

SparseMatrixCSC Float64.(A) drops structural nonzeros, but float(A) doesn't:

julia> A = SparseArrays.sparse([1 2 0; 0 0 3; 0 4 0])
julia> A[1, 1] = 0
julia> A
3×3 SparseArrays.SparseMatrixCSC{Int64, Int64} with 4 stored entries:
 0  2  ⋅
 ⋅  ⋅  3
 ⋅  4  ⋅

julia> Float64.(A)  # drops structural nonzeros !!
3×3 SparseArrays.SparseMatrixCSC{Float64, Int64} with 3 stored entries:
  ⋅   2.0   ⋅ 
  ⋅    ⋅   3.0
  ⋅   4.0   ⋅

julia> float(A) # OK ?!
3×3 SparseArrays.SparseMatrixCSC{Float64, Int64} with 4 stored entries:
 0.0  2.0   ⋅
  ⋅    ⋅   3.0
  ⋅   4.0   ⋅