allow data attributes on any links

On a project I wanted to add data attributes to all pagination links, then I customized all templates to add
    `:data => data`

And call pagination like

```ruby
paginate @user, data: { pjax: true }
```

This works.

However if I remove data attributes in `paginate` call, an error will thrown as "undefined local variable or method `data'"

Since data attributes is quite popular, I suggest to add a default value of `data: nil` in helper.