[SPARK-2052] [SQL] Add optimization for CaseConversionExpression's.

Add optimization for `CaseConversionExpression`'s.
Ok, I do.
BTW, I noticed that there are some Expressions in the same situation like `UnaryMinus`, `Cast`, or `Not`, ( or `BinaryArithmetic`, `BinaryComparison`, `StringRegexExpression`, too ? )
I think they also can be handled by constant folding.
What do you think?