[SPARK-18419][SQL] `JDBCRelation.insert` should not remove Spark options

## What changes were proposed in this pull request?

Currently, `JDBCRelation.insert` removes Spark options too early by mistakenly using `asConnectionProperties`. Spark options like `numPartitions` should be passed into `DataFrameWriter.jdbc` correctly. This bug have been **hidden** because `JDBCOptions.asConnectionProperties` fails to filter out the mixed-case options. This PR aims to fix both.

**JDBCRelation.insert**
```scala
override def insert(data: DataFrame, overwrite: Boolean): Unit = {
  val url = jdbcOptions.url
  val table = jdbcOptions.table
- val properties = jdbcOptions.asConnectionProperties
+ val properties = jdbcOptions.asProperties
  data.write
    .mode(if (overwrite) SaveMode.Overwrite else SaveMode.Append)
    .jdbc(url, table, properties)
```

**JDBCOptions.asConnectionProperties**
```scala
scala> import org.apache.spark.sql.execution.datasources.jdbc.JDBCOptions
scala> import org.apache.spark.sql.catalyst.util.CaseInsensitiveMap
scala> new JDBCOptions(Map("url" -> "jdbc:mysql://localhost:3306/temp", "dbtable" -> "t1", "numPartitions" -> "10")).asConnectionProperties
res0: java.util.Properties = {numpartitions=10}
scala> new JDBCOptions(new CaseInsensitiveMap(Map("url" -> "jdbc:mysql://localhost:3306/temp

", "dbtable" -> "t1", "numPartitions" -> "10"))).asConnectionProperties
res1: java.util.Properties = {numpartitions=10}
```

## How was this patch tested?

Pass the Jenkins with a new testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15863 from dongjoon-hyun/SPARK-18419.

(cherry picked from commit 55d528f2)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>