[Minor][SQL] Little refactor DataFrame related codes

Simplify some codes related to DataFrame.

*  Calling `toAttributes` instead of a `map`.
*  Original `createDataFrame` creates the `StructType` and its attributes in a redundant way. Refactored it to create `StructType` and call `toAttributes` on it directly.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4298 from viirya/refactor_df and squashes the following commits:

1d61c64 [Liang-Chi Hsieh] Revert it.
f36efb5 [Liang-Chi Hsieh] Relax the constraint of toDataFrame.
2c9f370 [Liang-Chi Hsieh] Just refactor DataFrame codes.