Change pluralize method to return the singular form of a word if the count is 0 and the last argument to true.

Hi, 

Currently with the ActionView::Helpers::TextHelper#pluralize method, we can't deal with the specific language rule about the zero value. For instance, in french, we use the singular form with the zero value:
- 0 cats <> 0 chat
- 1 cat == 1 chat
- 2 cats == 2 chats

This pull request tries to solve this problem.

Thanks.