Fix NoMethodError when calling unscope method above Rails 4

Example:

    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>
Merge branch 'estum-unscope-issue-rails-4'