fix: #290

[dajmcdon]

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• Make sure this PR is against "dev", not "main".
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  dajmcdon.

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• Resolves flatline_forecaster error when using target_date #290

[dajmcdon]

@dshemetov I think this fixes your issue. This is technically a bug that may be on main. I'm not entirely sure if it (alone) would fix main however (I could not reproduce the NA's on the dev branch, only the incorrect target date).

Update:
In testing on main, the NA's remain in .pred_distn if: ahead = 7L and target_date = "2021-01-01". This is not the case on dev, despite the fact that there is a mismatch between forecast_date + ahead and target_date.

It's probably worth adding a check to the *_args_list() constructors to ensure that this isn't misspecified.

[dshemetov]

Hm, there's still an issue when using n_training < ahead in flatline_forecaster, gonna address in another PR.