Optional distinct added for filtered_collection to sort nested associations

[Mirkoffcito]

Added an optional distinct for sorting nested associations with ransack gem, since you have to set .result(distinct: false) for it to work.

For example, given an endpoint where we have many users, and each user can have many posts, and each post have a title column:

• /users?q[s]=post_title asc should sort all users based on the title of their posts, but it won't work because of (distinct: true).
  But if we set distinct to false, it will sort properly.

This way, when we want to sort with an association, in out controller we would do:

• respond_with filtered_collection(User.all, false)