change_table temporal: true chokes when the base table has a computed index

[vjt]

It chokes with:

     NoMethodError:
       undefined method `join' for "lower(baz)":String
     Shared Example Group: "temporal table" called from ./spec/chrono_model/adapter/migrations_spec.rb:104
     # ./lib/chrono_model/adapter/indexes.rb:141:in `block (2 levels) in chrono_copy_indexes_to_history'
     # ./lib/chrono_model/adapter.rb:111:in `block in on_schema'
     # ./lib/chrono_model/adapter.rb:166:in `count_recursions'
     # ./lib/chrono_model/adapter.rb:106:in `on_schema'
     # ./lib/chrono_model/adapter.rb:93:in `on_history_schema'
     # ./lib/chrono_model/adapter/indexes.rb:138:in `block in chrono_copy_indexes_to_history'
     # ./lib/chrono_model/adapter/indexes.rb:135:in `each'
     # ./lib/chrono_model/adapter/indexes.rb:135:in `chrono_copy_indexes_to_history'
     # ./lib/chrono_model/adapter/migrations.rb:224:in `chrono_make_temporal_table'
     # ./lib/chrono_model/adapter/migrations.rb:75:in `block in change_table'
     # ./lib/chrono_model/adapter/migrations.rb:67:in `change_table'
     # ./spec/chrono_model/adapter/migrations_spec.rb:101:in `block (4 levels) in <top (required)>'