Evaluate ransackable_scopes before attributes when building the query

[faragorn]

This paragraph in README.md shows that Ransack can properly evaluate this scope:

 scope :salary_gt, ->(amount) { where('salary > ?', amount) }

if the scope is properly whitelisted:

  def self.ransackable_scopes(auth_object = nil)
    %i(salary_gt)
  end

However, it is not the case because attribute_method?(salary_gt) will evaluate to true even if the attribute is not whitelisted in ransackable/ransortable_attributes.

I assumed that scopes were introduced to easily override/customize #{attribute}_#{predicate} pair without breaking the naming convention.
In that case this PR should make ransack work as per the README mentioned above.

Test cases will follow, but I would appreciate initial Yes/No before putting more time in to it.

[scarroll32]

@faragorn it's been a while, but this looks very useful.

[faragorn]

@scarroll32 I will finish it hopefully this month

[deivid-rodriguez]

This one also makes sense to me!

[scarroll32]

@faragorn this is a candidate for the 3.0.0 Release if you can finish it. 🥇

[faragorn]

@scarroll32 Added a spec, and rebased, let me know if looks good and further steps. Cheers!

[scarroll32]

Thank you for your contribution @faragorn, it will be pushed to RubyGems as part of 3.0.0, and available as a ref to master until then.