[SPARK-11679][SQL] Invoking method " apply(fields:...

[SPARK-11679][SQL] Invoking method " apply(fields: java.util.List[StructField])" in "StructType" gets ClassCastException

In the previous method, fields.toArray will cast java.util.List[StructField] into Array[Object] which can not cast into Array[StructField], thus when invoking this method will throw "java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Lorg.apache.spark.sql.types.StructField;"
I directly cast java.util.List[StructField] into Array[StructField]  in this patch.

Author: mayuanwen <mayuanwen@qiyi.com>

Closes #9649 from jackieMaKing/Spark-11679.