Add a failing test to demonstrate a bug

[mjnaderi]

We found a bug in our project, and after investigating, we found where it comes from. So I created a failing test based on our issue.

In this test, removing prefetch_related from this line:

my_teams = Team.objects.filter(user_profiles=user_c).prefetch_related('user_profiles').distinct()

or removing this "for" loop:

for _ in my_teams:
    pass

resolves the issue, and test passes. But with both of them, test fails.

[mjnaderi]

Another note:
The behavior is different with Sqlite and PostgreSql.

With Sqlite, the test fails with:

Traceback (most recent call last):
  File "/home/naderi/Repos/django-polymorphic/polymorphic/tests.py", line 1379, in test_unknown_issue
    self.assertEqual(len(my_teams[0].user_profiles.all()), 3)
AssertionError: 4 != 3

With PostgreSql, the test fails with:

Traceback (most recent call last):
  File "/home/naderi/Repos/django-polymorphic/polymorphic/tests.py", line 1379, in test_unknown_issue
    self.assertEqual(len(my_teams[0].user_profiles.all()), 3)
AssertionError: 2 != 3

[mjnaderi]

Now I found issue #68 and pull request #250. Maybe this issue is related to them.

[WhyNotHugo]

This doesn't look right; you're not ordering you QuerySet, so the first result returned by the DB might be team2. It'll probably fail, occasionally.

[bckohan]

This PR has been stuck in this state for the last day, so I have pulled the commits and am superceding it with #671