c2d does not maintain type #441
Description
Noticed that when using c2d on a zpk system it changes type in the returned system. Seems like it could be nice to have the type stay the same.
julia> G = zpk([], [-1.0], 1)
TransferFunction{Continuous, ControlSystems.SisoZpk{Float64, Float64}}
1.0
1.0----------
1.0s + 1.0
Continuous-time transfer function model
julia> c2d(G, 0.1)
TransferFunction{Discrete{Float64}, ControlSystems.SisoRational{Float64}}
0.09516258196404037
-------------------------
1.0z - 0.9048374180359594
Sample Time: 0.1 (seconds)
Discrete-time transfer function model