## What changes were proposed in this pull request?
This pr added `unionByName` in `DataSet`.
Here is how to use:
```
val df1 = Seq((1, 2, 3)).toDF("col0", "col1", "col2")
val df2 = Seq((4, 5, 6)).toDF("col1", "col2", "col0")
df1.unionByName(df2).show
// output:
// +----+----+----+
// |col0|col1|col2|
// +----+----+----+
// | 1| 2| 3|
// | 6| 4| 5|
// +----+----+----+
```
## How was this patch tested?
Added tests in `DataFrameSuite`.
Author: Takeshi Yamamuro <yamamuro@apache.org>
Closes#18300 from maropu/SPARK-21043-2.
a2bec6c92a