Fix deprecation warning when showing error with rails 6.

[madleech]

This fixes the following error when using Rails 6:

ActiveRecord::Base.configurations in Rails 6 now returns an object instead of a hash. The keys method is not supported. Please use configs_for or consult the documentation for supported methods.

Context:

/Users/michael/.rvm/gems/ruby-2.5.1/gems/activerecord-6.0.3.3/lib/active_record/database_configurations.rb:221:in `method_missing': `ActiveRecord::Base.configurations` in Rails 6 now returns an object instead of a hash. The `keys` method is not supported. Please use `configs_for` or consult the documentation for supported methods. (NotImplementedError)
  1: from /Users/michael/.rvm/gems/ruby-2.5.1/gems/activerecord-migrations-1.5.0/lib/active_record/migrations/tasks.rb:45:in `each_current_configuration'
  2: from /Users/michael/.rvm/gems/ruby-2.5.1/gems/activerecord-6.0.3.3/lib/active_record/tasks/database_tasks.rb:207:in `drop_current'

... which happens when RAILS_ENV and DATABASE_VARIANT disagree with each other.

Added a unit test to show a possible invocation.

[ddoherty03]

Any chance of getting this merged into master? I'm getting this after upping to AR6 for a non-rails app.