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]

Codecov Report

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@@          Coverage Diff           @@
##             dev     #506   +/-   ##
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  Coverage       ?   84.60%           
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  Files          ?       31           
  Lines          ?     3157           
  Branches       ?        0           
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  Hits           ?     2671           
  Misses         ?      486           
  Partials       ?        0           

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