Fix. some missing attribute on PassThroughEncoder

Fixes #1583.
Apparently, this error was showing up (https://github.com/numenta/nupic/issues/1583):

  Traceback (most recent call last):
    File "train_cla.py", line 87, in <module>
      main()
    File "train_cla.py", line 73, in main
      inferences = recogniter.run(input_data, learn=True, class_learn=True, learn_layer=None)
    File "/Users/lmj/project/env/nupic/recognition/nupic_dir/lib/cla_classifier.py", line 265, in run
      self.network.run(1)
    File "/Users/lmj/project/env/nupic/nupic/nupic/engine/__init__.py", line 669, in run
      engine.Network.run(self, n)
    File "/Users/lmj/project/env/nupic/nupic/nupic/bindings/engine_internal.py", line 1141, in run
      return _engine_internal.Network_run(self, *args, **kwargs)
    File "/Users/lmj/project/env/nupic/nupic/nupic/regions/RecordSensor.py", line 297, in compute
      self._outputValues['sourceOut'] = self.encoder.getEncodedValues(data)
    File "/Users/lmj/project/env/nupic/nupic/nupic/encoders/base.py", line 316, in getEncodedValues
      values = encoders.getEncodedValues(self._getInputValue(inputData, name))
    File "/Users/lmj/project/env/nupic/nupic/nupic/encoders/base.py", line 314, in getEncodedValues
      if self.encoders is not None:
  AttributeError: 'SparsePassThroughEncoder' object has no attribute 'encoders'