not use assert_not_nil in Errors#[]

r7593 etc. replaced Rails2 Errors#on.
Rails3 Errors#[] always return array.
So, Rails3 Errors#[] is always not nil.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12070 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/unit/issue_nested_set_test.rb

@@ -57,7 +57,7 @@ class IssueNestedSetTest < ActiveSupport::TestCase
     child = Issue.new(:project_id => 2, :tracker_id => 1, :author_id => 1,
                       :subject => 'child', :parent_issue_id => issue.id)
     assert !child.save
-    assert_not_nil child.errors[:parent_issue_id]
+    assert_not_equal [], child.errors[:parent_issue_id]
   end
 
   def test_move_a_root_to_child
@@ -163,7 +163,7 @@ class IssueNestedSetTest < ActiveSupport::TestCase
     child.reload
     child.parent_issue_id = grandchild.id
     assert !child.save
-    assert_not_nil child.errors[:parent_issue_id]
+    assert_not_equal [], child.errors[:parent_issue_id]
   end
 
   def test_destroy_should_destroy_children