Fixed BacktrackingTM serialization error

[rhyolight]

Fixes #3763

[numenta-ci]

@rhyolight, thanks for your PR! By analyzing the history of the files in this pull request, we identified @scottpurdy, @marionleborgne and @subutai to be potential reviewers.

[rhyolight]

@lscheinkman

[lscheinkman]

@rhyolight This class serialization is currently not covered by our tests (see https://ci.numenta.com/browse/NUP-PY-561/artifact/TL/HTML-Coverage-Report/_root_bamboo-agent-home_xml-data_build-dir_NUP-PY-TL_src_nupic_algorithms_backtracking_tm.html#n1372). That's why it remained unnoticed. Maybe we could also add a test covering this issue, either in this PR or in a separate PR.

[rhyolight]

@lscheinkman I'd be more than happy if someone did this, but honestly increasing test coverage on this code base is not a high priority. If anyone reading this wants to create an issue and a test to cover this bug, please do.