[SPARK-15811][SQL] fix the Python UDF in Scala 2.10

## What changes were proposed in this pull request?

Iterator can't be serialized in Scala 2.10, we should force it into a array to make sure that .

## How was this patch tested?

Build with Scala 2.10 and ran all the Python unit tests manually (will be covered by a jenkins build).

Author: Davies Liu <davies@databricks.com>

Closes #13717 from davies/fix_udf_210.