[SPARK-15190][SQL] Support using SQLUserDefinedType for case classes

## What changes were proposed in this pull request?

Right now inferring the schema for case classes happens before searching the SQLUserDefinedType annotation, so the SQLUserDefinedType annotation for case classes doesn't work.

This PR simply changes the inferring order to resolve it. I also reenabled the java.math.BigDecimal test and added two tests for `List`.

## How was this patch tested?

`encodeDecodeTest(UDTCaseClass(new java.net.URI("http://spark.apache.org/")), "udt with case class")`

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #12965 from zsxwing/SPARK-15190.