`Cast.nullable` should be `true` when cast from `StringType` to `NumericType` or `TimestampType`.
Because if `StringType` expression has an illegal number string or illegal timestamp string, the casted value becomes `null`.

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

Closes #532 from ueshin/issues/SPARK-1608 and squashes the following commits:

065d37c [Takuya UESHIN] Add tests to check nullabilities of cast expressions.
f278ed7 [Takuya UESHIN] Revert test to keep it readable and concise.
9fc9380 [Takuya UESHIN] Fix Cast.nullable when cast from StringType to NumericType/TimestampType.