[SPARK-16934][ML][MLLIB] Update LogisticCostAggregator serialization code to...

[SPARK-16934][ML][MLLIB] Update LogisticCostAggregator serialization code to make it consistent with LinearRegression

## What changes were proposed in this pull request?

Update LogisticCostAggregator serialization code to make it consistent with #14109

## How was this patch tested?
MLlib 2.0:
![image](https://cloud.githubusercontent.com/assets/19235986/17649601/5e2a79ac-61ee-11e6-833c-3bd8b5250470.png)

After this PR:
![image](https://cloud.githubusercontent.com/assets/19235986/17649599/52b002ae-61ee-11e6-9402-9feb3439880f.png)

Author: WeichenXu <WeichenXu123@outlook.com>

Closes #14520 from WeichenXu123/improve_logistic_regression_costfun.