[SPARK-20274][SQL] support compatible array element type in encoder

## What changes were proposed in this pull request?

This is a regression caused by SPARK-19716.

Before SPARK-19716, we will cast an array field to the expected array type. However, after SPARK-19716, the cast is removed, but we forgot to push the cast to the element level.

## How was this patch tested?

new regression tests

Author: Wenchen Fan <wenchen@databricks.com>

Closes #17587 from cloud-fan/array.