Fix attribute when converting factors in ChoicesFilterState

[nissinbo]

Fixes #611

droplevels() was applied to avoid displaying unused levels in the filter. This dropped the attributes.
As a workaround, this PR restores the attributes after applying droplevels(). Please let me know if you would prefer a different approach.

[github-actions]

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[nissinbo]

I have read the CLA Document and I hereby sign the CLA

[osenan]

Hi @nissinbo thanks for your contribution. I was working on this issue and arrived to the same solution that yours. I've also added some unit tests to check this functionality. Therefore we will continue reviewing the other PR, but basically we reached the same fix!

[nissinbo]

@osenan Thank you! This seems to be resolved by #679, so I will close this PR.