[SPARK-2254] [SQL] ScalaRefection should mark primitive types as non-nullable.

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

Closes #1193 from ueshin/issues/SPARK-2254 and squashes the following commits:

cfd6088 [Takuya UESHIN] Modify ScalaRefection.schemaFor method to return nullability of Scala Type.