message: Make TF2_BEHAVIOR=0 disable TF2 behavior.
  Prior to this change, the mere presence of a TF2_BEHAVIOR
  environment variable would enable TF2 behavior. With this,
  setting that environment variable to "0" will disable it.
diff: @@ -42,4 +42,4 @@ def disable():
  def enabled():
  """Returns True iff TensorFlow 2.0 behavior should be enabled."""
  - return _force_enable or os.getenv("TF2_BEHAVIOR") is not None
  + return _force_enable or os.getenv("TF2_BEHAVIOR", "0") != "0"
  