[SPARK-2431][SQL] Refine StringComparison and related codes.

Refine `StringComparison` and related codes as follows:
- `StringComparison` could be similar to `StringRegexExpression` or `CaseConversionExpression`.
- Nullability of `StringRegexExpression` could depend on children's nullabilities.
- Add a case that the like condition includes no wildcard to `LikeSimplification`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1357 from ueshin/issues/SPARK-2431 and squashes the following commits:

77766f5 [Takuya UESHIN] Add a case that the like condition includes no wildcard to LikeSimplification.
b9da9d2 [Takuya UESHIN] Fix nullability of StringRegexExpression.
680bb72 [Takuya UESHIN] Refine StringComparison.
[SPARK-2415] [SQL] RowWriteSupport should handle empty ArrayType correctly.

`RowWriteSupport` doesn't write empty `ArrayType` value, so the read value becomes `null`.
It should write empty `ArrayType` value as it is.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1339 from ueshin/issues/SPARK-2415 and squashes the following commits:

32afc87 [Takuya UESHIN] Merge branch 'master' into issues/SPARK-2415
2f05196 [Takuya UESHIN] Fix RowWriteSupport to handle empty ArrayType correctly.
[SPARK-2428][SQL] Add except and intersect methods to SchemaRDD.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1355 from ueshin/issues/SPARK-2428 and squashes the following commits:

b6fa264 [Takuya UESHIN] Add except and intersect methods to SchemaRDD.