Non-deterministic test failure in test_count_objects

This looks like a non-deterministic test failure:

```
======================================================================
FAIL: test_count_objects (dozer.tests.test_reftree.TestGlobals)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/travis/build/mgedmin/dozer/dozer/tests/test_reftree.py", line 61, in test_count_objects
self.assertTrue((1, MyObj) in res)
AssertionError: False is not true
```

-- https://travis-ci.org/mgedmin/dozer/jobs/192265786

Start with dozer/tests/test_reftree.py, especially test_count_objects at line 61, and compare the failure with the Travis CI job linked in the issue. Reproduce the test failure and investigate why (1, MyObj) is absent from res; done means the test is reliable and passes consistently.

Written by the indexing model from the issue text.