[SPARK-17369][SQL] MetastoreRelation toJSON throws AssertException due to missing otherCopyArgs

## What changes were proposed in this pull request?

`TreeNode.toJSON` requires a subclass to explicitly override otherCopyArgs to include currying construction arguments, otherwise it reports AssertException telling that the construction argument values' count doesn't match the construction argument names' count.

For class `MetastoreRelation`, it has a currying construction parameter `client: HiveClient`, but Spark forgets to add it to the list of otherCopyArgs.

## How was this patch tested?

Unit tests.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #14928 from clockfly/metastore_relation_toJSON.