When added multiple counter (with filtering), tests failed.

[moumoutte]

This patch make tests failed (in my own machin),

http://paste.debian.net/176840/

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FAIL: test_real_triggers (django_aggtrigg_demo.tests.TestMockingTrigger)
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Traceback (most recent call last):
  File "/home/moumoutte/projects/django-aggtrigg/demo/django_aggtrigg_demo/tests.py", line 58, in      test_real_triggers
    Tree.objects.get_count().first().leave_count_private_leaves, 5)
AssertionError: 0 != 5

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Ran 7 tests in 1.310s