[SPARK-11379][SQL] ExpressionEncoder can't handle top level primitive type correctly

For inner primitive type(e.g. inside `Product`), we use `schemaFor` to get the catalyst type for it, https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala#L403.

However, for top level primitive type, we use `dataTypeFor`, which is wrong.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #9337 from cloud-fan/encoder.