fix site title of namespaces

Bug:
With this configuration:
```ruby
ActiveAdmin.setup do |config|
  config.site_title = "Root"
  config.namespace :sub do |sub|
    sub.site_title = "Sub"
  end
end
```
The site title (`<title>` tag), keeps "Root" for all namespaces. Now it's "Root" for root namespace and "Sub" for sub namespace.