zero & keytype always return Int64 for the index type

```julia
julia> using SparseArrays

julia> v=spzeros(Int,Int32,10)
10-element SparseVector{Int64, Int32} with 0 stored entries

julia> keytype(v)
Int64

julia> zero(v)
10-element SparseVector{Int64, Int64} with 0 stored entries
```