message: fix test for target_column type
diff: @@ -82,7 +82,7 @@ class PandasIoTest(test.TestCase):
  x, y = self.makeTestDataFrame()
  with self.assertRaisesRegexp(TypeError,
  - 'target_column must be a string or None'):
  + 'target_column must be a string type'):
  pandas_io.pandas_input_fn(x, y, batch_size=2,
  shuffle=False,
  num_epochs=1,
  