[SPARK-4618][SQL] Make foreign DDL commands options case-insensitive

Using lowercase for ```options``` key to make it case-insensitive, then we should use lower case to get value from parameters.
So flowing cmd work
```
      create temporary table normal_parquet
      USING org.apache.spark.sql.parquet
      OPTIONS (
        PATH '/xxx/data'
      )
```

Author: scwf <wangfei1@huawei.com>
Author: wangfei <wangfei1@huawei.com>

Closes #3470 from scwf/ddl-ulcase and squashes the following commits:

ae78509 [scwf] address comments
8f4f585 [wangfei] address comments
3c132ef [scwf] minor fix
a0fc20b [scwf] Merge branch 'master' of https://github.com/apache/spark into ddl-ulcase
4f86401 [scwf] adding CaseInsensitiveMap
e244e8d [wangfei] using lower case in json
e0cb017 [wangfei] make options in-casesensitive