2-1-stable: fix ActiveRecord::StatementInvalid: Error: Column 'lft' in order clause is ambiguous

On 2-1-stable 3d5ac746542, Redmine test fails.
https://bitbucket.org/marutosi/redmine-bb-awesome_nested_set-use-gem/src/2ac1078370/Gemfile#cl-10
https://drone.io/bitbucket.org/marutosi/redmine-bb-awesome_nested_set-use-gem/2

<pre>
 13) Error:
    test_visible_and_nested_set_scopes(IssueTest):
    ActiveRecord::StatementInvalid: Mysql2::Error: Column 'lft' in order clause is ambiguous:
</pre>

http://www.redmine.org/projects/redmine/repository/revisions/12689/annotate/trunk/test/unit/issue_test.rb#L332
<pre>
  def test_visible_and_nested_set_scopes
    assert_equal 0, Issue.find(1).descendants.visible.all.size
  end
</pre>
Merge pull request #235 from marutosi/2-1-stable-fix-order

2-1-stable: fix ActiveRecord::StatementInvalid: Error: Column 'lft' in order clause is ambiguous