## What changes were proposed in this pull request?
This patch fixes a longstanding issue with one of the RelationalGroupedDataset.agg function. Even though the signature accepts vararg of pairs, the underlying implementation turns the seq into a map, and thus not order preserving nor allowing multiple aggregates per column.
This change also allows users to use this function to run multiple different aggregations for a single column, e.g.
```
agg("age" -> "max", "age" -> "count")
```
## How was this patch tested?
Added a test case in DataFrameAggregateSuite.
Author: petermaxlee <petermaxlee@gmail.com>
Closes#14697 from petermaxlee/SPARK-17124.
9560c8d295