Added sample test code to reproduce issue #68

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This is just an attempt to deal with #68.

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I took some time yesterday to track down the issue, and here is what I got:

1. Apparently, prefetch_related on the queryset calls prefetch_related_objects (https://github.com/django/django/blob/1.10.1/django/db/models/query.py#L1387)
2. This function calls prefetch_one_level (https://github.com/django/django/blob/1.10.1/django/db/models/query.py#L1473)
3. At some point, this function retrieve a queryset from the prefetcher get_prefetch_queryset instance (https://github.com/django/django/blob/1.10.1/django/db/models/query.py#L1580). If I'm right, this queryset of related objects is then looped over and results are binded to original requested instances
4. The mess seems to happen exactly here: to retrieve the related objects, an extra query is used by the get_prefetch_queryset method, and it is in this method I found the wrong PK's of the related objects to be returned.

I'll take more time to clearly identify if the line linked in 4 is where the error happens, because this ORM code is full of lambdas, descriptors and other deep-vodoo-black-magic stuff that are not easy to grasp. However, this extra query seems like a good culprit to me.