Fix `append` for delay system

[baggepinnen]

closes #879

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b3ffc72) 31.57% compared to head (3d4cc67) 31.57%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #892 +/- ## ======================================= Coverage 31.57% 31.57% ======================================= Files 43 43 Lines 4624 4624 ======================================= Hits 1460 1460 Misses 3164 3164 ``` | [Files](https://app.codecov.io/gh/JuliaControl/ControlSystems.jl/pull/892?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaControl) | Coverage Δ | | |---|---|---| | [lib/ControlSystemsBase/src/types/LFTSystem.jl](https://app.codecov.io/gh/JuliaControl/ControlSystems.jl/pull/892?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaControl#diff-bGliL0NvbnRyb2xTeXN0ZW1zQmFzZS9zcmMvdHlwZXMvTEZUU3lzdGVtLmps) | `84.28% <100.00%> (ø)` | |

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