ActiveSupport::Inflector.underscore messes up acronyms under namespaces

For example:

```ruby
ActiveSupport::Inflector.inflections do |inflect|
  inflect.acronym 'ISP'
  inflect.acronym 'ISPs'
end

"ISP".pluralize.underscore 
  => "isps" (ok)
"SomeISP".pluralize.underscore 
  => "some_isps" (ok)
"Provider::ISP".pluralize.underscore
  => "provider/is_ps" (should be provider/isps)
```