Custom DNS setting is lost when you change to a built-in server

[cjhenck]

When you change from a custom DNS provider to a built-in server, the custom provider is not remembered. It may be useful to retain it in case it's a favored setting.

Steps to reproduce:

1. Switch to a custom provider and save
2. Switch settings back to a built-in provider and save
3. Settings page no longer has content in the custom provider field.