Major Description
if we have a DataFrame with schema a: int, b: int, c: int, and convert it to Dataset with case class Data(a: Int, c: Int), it works and we will extract the `a` and `c` columns to build the Data.
However, if the struct is inside array, e.g. schema is arr: array<struct<a: int, b: int, c: int>>, and we wanna convert it to Dataset with case class ComplexData(arr: Seq[Data]), we will fail. The reason is, to allow compatible types, e.g. convert a: int to case class A(a: Long), we will add cast for each field, except struct type field, because struct type is flexible, the number of columns can mismatch. We should probably also skip cast for array and map type.
