Add support for attribute alias

[ghost]

See #589

[jonatack]

We could probably just inline the values here without memoizing to a local var:

def ransackable_attributes(auth_object = nil)
  if Ransack::SUPPORTS_ATTRIBUTE_ALIAS
    column_names + _ransackers.keys + attribute_aliases.keys
  else
    column_names + _ransackers.keys
  end
end

[jonatack]

Thanks! This looks pretty good. Can you let me know for the question above?

[ghost]

Inlined the things in the ransackable_attributes method.

[jonatack]

Thanks! 🍏 Merged here.

[jonatack]

Would you like to make a PR to update the change log?

[ghost]

@jonatack: see #594