[SPARK-18433][SQL] Improve DataSource option keys to be more case-insensitive

## What changes were proposed in this pull request?

This PR aims to improve DataSource option keys to be more case-insensitive

DataSource partially use CaseInsensitiveMap in code-path. For example, the following fails to find url.

```scala
val df = spark.createDataFrame(sparkContext.parallelize(arr2x2), schema2)
df.write.format("jdbc")
    .option("UrL", url1)
    .option("dbtable", "TEST.SAVETEST")
    .options(properties.asScala)
    .save()
```

This PR makes DataSource options to use CaseInsensitiveMap internally and also makes DataSource to use CaseInsensitiveMap generally except `InMemoryFileIndex` and `InsertIntoHadoopFsRelationCommand`. We can not pass them CaseInsensitiveMap because they creates new case-sensitive HadoopConfs by calling newHadoopConfWithOptions(options) inside.

## How was this patch tested?

Pass the Jenkins test with newly added test cases.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15884 from dongjoon-hyun/SPARK-18433.

(cherry picked from commit 74f5c217)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>