Code review comment for lp:~codersquid/python-oops-datedir-repo/optional-prune-dependencies

Using run-of-the-mill unittest, trunk has a failure, if I have time I'll take a look at it to get things clean before proceeding with work on this branch.

virtualenv venv
. venv/bin/activate
pip install -e .\[test\]
venv/bin/python -m unittest discover -v

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ERROR: test_read_detect_rfc822 (oops_datedir_repo.tests.test_serializer.TestParsing)
oops_datedir_repo.tests.test_serializer.TestParsing.test_read_detect_rfc822
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_StringException: Traceback (most recent call last):
  File "oops_datedir_repo/tests/test_serializer.py", line 63, in test_read_detect_rfc822
    self.assertEqual(self.expected_dict, read(source_file))
  File "oops_datedir_repo/serializer.py", line 61, in read
    return serializer_bson.read(StringIO(content))
  File "oops_datedir_repo/serializer_bson.py", line 57, in read
    report = bson.loads(fp.read())
  File "/home/codersquid/work/hexr/python-oops-datedir-repo/venv/local/lib/python2.7/site-packages/bson/__init__.py", line 47, in loads
    return decode_document(data, 0)[1]
  File "/home/codersquid/work/hexr/python-oops-datedir-repo/venv/local/lib/python2.7/site-packages/bson/codec.py", line 269, in decode_document
    if data[end_point - 1] not in ('\0', 0):
IndexError: string index out of range