message: add test for gen_item_tensor_string
diff: @@ -46,6 +46,13 @@ class SlicesTest(test.TestCase):
  with self.test_session() as sess:
  self.assertAllEqual(sess.run(t), [3, 4])
  + def test_get_item_tensor_string(self):
  + initial_str = constant_op.constant("abcd")
  + t = slices.get_item(initial_str, 1, slices.GetItemOpts(element_dtype=initial_str.dtype))
  +
  + with self.test_session() as sess:
  + self.assertEqual(sess.run(t), b"b")
  +
  if __name__ == '__main__':
  test.main()
  