[SPARK-8283][SQL] CreateStruct should not specify the field names

[chenghao-intel]

CreateStruct <=> GenericUDFStruct which always give the default column names for the output struct like (col1, col2...colN)

[chenghao-intel]

@YijieSHEN

[yjshen]

I prefer to keep the origin column names in the newly created struct, since I think it's more meaningful than col1, col2, col3, and we could just leave the unnamed columns to col1, col2 ..., which is also compatible with Hive's semantic.

I've also made related changes in #6874 to loosen parameter requirements of struct

[chenghao-intel]

Ok, sound reasonable to me, closing this PR.

[SparkQA]

Test build #35127 has finished for PR 6881 at commit 35fa5fb.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #35130 has finished for PR 6881 at commit 2efe8ba.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.