[SPARK-2093] [SQL] NullPropagation should use exact type value.

`NullPropagation` should use exact type value when transform `Count` or `Sum`.

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

Closes #1034 from ueshin/issues/SPARK-2093 and squashes the following commits:

65b6ff1 [Takuya UESHIN] Modify the literal value of the result of transformation from Sum to long value.
830c20b [Takuya UESHIN] Add Cast to the result of transformation from Count.
9314806 [Takuya UESHIN] Fix NullPropagation to use exact type value.

(cherry picked from commit 0402bd77ec786d1fa6cfd7f9cc3aa97c7ab16fd8)
Signed-off-by: Michael Armbrust <michael@databricks.com>