[SPARK-4270][SQL] Fix Cast from DateType to DecimalType.

`Cast` from `DateType` to `DecimalType` throws `NullPointerException`.

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

Closes #3134 from ueshin/issues/SPARK-4270 and squashes the following commits:

7394e4b [Takuya UESHIN] Fix Cast from DateType to DecimalType.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala

@@ -281,7 +281,7 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression w
     case BooleanType =>
       buildCast[Boolean](_, b => changePrecision(if (b) Decimal(1) else Decimal(0), target))
     case DateType =>
-      buildCast[Date](_, d => changePrecision(null, target)) // date can't cast to decimal in Hive
+      buildCast[Date](_, d => null) // date can't cast to decimal in Hive
     case TimestampType =>
       // Note that we lose precision here.
       buildCast[Timestamp](_, t => changePrecision(Decimal(timestampToDouble(t)), target))

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala

@@ -412,6 +412,8 @@ class ExpressionEvaluationSuite extends FunSuite {
     checkEvaluation(Cast(d, LongType), null)
     checkEvaluation(Cast(d, FloatType), null)
     checkEvaluation(Cast(d, DoubleType), null)
+    checkEvaluation(Cast(d, DecimalType.Unlimited), null)
+    checkEvaluation(Cast(d, DecimalType(10, 2)), null)
     checkEvaluation(Cast(d, StringType), "1970-01-01")
     checkEvaluation(Cast(Cast(d, TimestampType), StringType), "1970-01-01 00:00:00")
   }