Adding paginated? helper to active record extensions and page scope methods

[jarenas9539]

Main Purpose

Adding a paginated? helper to PageScopeMethods and AR extensions.

Context

Lately I found myself implementing a See All option in a few pages. As expected I had to find a way to conditionally render paginate section leading to conditions on views as relation.respond_to?(:total_pages) or relation.limit_value.present?.
That was the moment when the paginated? helper came to my mind, a simple and clean method to achieve this check.

Changes

• adding paginated? with default value false to AR relations to have it available even if pagination method wasn't called
• defining paginated? on PageScopeMethods relying on limit_value (this way we ensure the helper being true just when the pagination method is called and include support to possible unscope calls too)
• added tests ✅
• updated README

On a separated note: contributions section on README should be updated, current lines described there to run specs are relying on removed gemfiles/*

[yuki24]

Looks good to me 👍

@amatsuda What do you think of this change?