[SPARK-10480] [ML] Fix ML.LinearRegressionModel.copy()

This PR fix two model ```copy()``` related issues:
[SPARK-10480](https://issues.apache.org/jira/browse/SPARK-10480)
```ML.LinearRegressionModel.copy()``` ignored argument ```extra```, it will not take effect when users setting this parameter.
[SPARK-10479](https://issues.apache.org/jira/browse/SPARK-10479)
```ML.LogisticRegressionModel.copy()``` should copy model summary if available.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #8641 from yanboliang/linear-regression-copy.