(hotfix) eliminate assertIsInstance in NN test

Remove unit test feature that is not available in python 2.6.

Reason why we need to treat this pull-req as hotfix:
* We want to support Python 2.6 in 0.5.0; assertIsInstance prevents running CI tests against python 2.6.: http://ci.jubat.us/job/jubatus-python-client/Branch=master,Python=2.6.6/