[SPARK-18604][SQL] Make sure CollapseWindow returns the attributes in the same order.

## What changes were proposed in this pull request?
The `CollapseWindow` optimizer rule changes the order of output attributes. This modifies the output of the plan, which the optimizer cannot do. This also breaks things like `collect()` for which we use a `RowEncoder` that assumes that the output attributes of the executed plan are equal to those outputted by the logical plan.

## How was this patch tested?
I have updated an incorrect test in `CollapseWindowSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #16027 from hvanhovell/SPARK-18604.

(cherry picked from commit 454b8049)
Signed-off-by: Herman van Hovell <hvanhovell@databricks.com>