[FIX]the estimator generate by tf.keras.model_to_estimator() cannot export...

[FIX]the estimator generate by tf.keras.model_to_estimator() cannot export saved_model because the model_fn provided by _create_keras_model_fn wasn't set export_outputs in the returned EstimatorSpec. Here I provide a default export_outputs with serve_default key and Predict API, and the result inside is same as predictions
[FIX]_save_first_checkpoint call saver.save with only a path and without filename, that make the ckpt saved with name like `{model_dir}/.meta` and `{model_dir}/.index`, which can not be found by latest_checkpoint("{model_dir}"). As state by save method of Saver, save_path should be a path to the checkpoint name. So to fix this, I change the name to `{model_dir}/keras_model.ckpt`