robotidy --separator tab does not respect tabs in Settings

When using robotidy on robot framework file that has a Settings section containing tabs, it replaces tabs with space even if --separator tab is used.

Reproduce the issue with a Robot Framework file whose Settings section contains tabs, using robotidy with --separator tab. Trace the formatting path for the Settings section and add a regression test; done means tabs in Settings remain tabs when that option is used.

Written by the indexing model from the issue text.