[SPARK-14402][SQL] initcap UDF doesn't match Hive/Oracle behavior in lowercasing rest of string

## What changes were proposed in this pull request?

Current, SparkSQL `initCap` is using `toTitleCase` function. However, `UTF8String.toTitleCase` implementation changes only the first letter and just copy the other letters: e.g. sParK --> SParK. This is the correct implementation `toTitleCase`.
```
hive> select initcap('sParK');
Spark
```
```
scala> sql("select initcap('sParK')").head
res0: org.apache.spark.sql.Row = [SParK]
```

This PR updates the implementation of `initcap` using `toLowerCase` and `toTitleCase`.

## How was this patch tested?

Pass the Jenkins tests (including new testcase).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12175 from dongjoon-hyun/SPARK-14402.