[SPARK-18123][SQL] Use db column names instead of RDD column ones during JDBC Writing

## What changes were proposed in this pull request?

Apache Spark supports the following cases **by quoting RDD column names** while saving through JDBC.
- Allow reserved keyword as a column name, e.g., 'order'.
- Allow mixed-case colume names like the following, e.g., `[a: int, A: int]`.

  ``` scala
  scala> val df = sql("select 1 a, 1 A")
  df: org.apache.spark.sql.DataFrame = [a: int, A: int]
  ...
  scala> df.write.mode("overwrite").format("jdbc").options(option).save()
  scala> df.write.mode("append").format("jdbc").options(option).save()
  ```

This PR aims to use **database column names** instead of RDD column ones in order to support the following additionally.
Note that this case succeeds with `MySQL`, but fails on `Postgres`/`Oracle` before.

``` scala
val df1 = sql("select 1 a")
val df2 = sql("select 1 A")
...
df1.write.mode("overwrite").format("jdbc").options(option).save()
df2.write.mode("append").format("jdbc").options(option).save()
```
## How was this patch tested?

Pass the Jenkins test with a new testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>
Author: gatorsmile <gatorsmile@gmail.com>

Closes #15664 from dongjoon-hyun/SPARK-18123.

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