Fix NoMethodError when calling unscope method

Example:

```ruby
class User < ActiveRecord::Base
  scope :with_activity, ->{ where{(active == true) & (posts_count + comments_count >= 1)} }
end

User.with_activity.unscope(where: :active)
# => NoMethodError: undefined method `symbol' for #<Squeel::Nodes::Operation:0x00000018bbf798>
#    from gems/squeel-1.2.1/lib/squeel/adapters/active_record/4.0/relation_extensions.rb:44:in `block in where_unscoping'
```