Fix: Allow use of symbol for configuration `top_namespace`

[unused]

Using a top_namespace in configuration that is a symbol fails with further processing where a string is expected. Relying on a conversion in settings constructor prevents an unexpected error.

What error is happening?

Given a symbol is used for top_namespace, it results in the following error:

Namespaced messages with top namespace allows symbol for top namespace
     Failure/Error: data.transform_keys { _1.gsub(DEFAULT_MESSAGES_ROOT, config.top_namespace) }

     TypeError:
       no implicit conversion of Symbol into String

[!NOTE] Ref: Related issue 734 in dry-validation