zpk view of `NamedStateSpace` structure

[B-LIE]

It is not a function I use a lot, but it seems like the zpk view of linear systems in NamedStateSpace structure does not work. tf does work.

test = ss([-1 2 3; 0 -2 3; 0 0 -3],[1;2;3],[1 2 3], [0.0])

tf(test)

tf(named_ss(test))

zpk(test)

This works (however, tf does not work if all numbers are integers -- e.g., if the D-matrix of test is set to [0], tf fails.).

The following does not work:

zpk(named_ss(test))

[baggepinnen]

Thanks for the issue, it's closed by #847 and https://github.com/JuliaControl/ControlSystems.jl/commit/20365704630380bc739f134c0ffa379009999459