[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'
      )
``` 
`spark.sql.caseSensitive` is about identifiers (i.e.,  attributes and table names).  I'd say this is more analogous to keyword case insensitivity.  I don't know any database that doesn't treat `SELECT` and `select` the same so I'm not sure if that should be configurable.

You can still pass your `CaseInsensitiveMap ` in and it will have the desired effect.  Just don't change the function signature.