fixed promotion for delay systems

[mfalt]

Minor fix where promotion from TransferFunction didn't work correctly.

[JuliaControlBot]

Something failed when generating plots. See the log at https://github.com/JuliaControl/ControlExamplePlots.jl/runs/841840289?check_suite_focus=true for more details.

[codecov[bot]]

Codecov Report

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  Hits           ?     2671           
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