[SPARK-8342][SQL] Fix Decimal setOrNull

JIRA: https://issues.apache.org/jira/browse/SPARK-8342

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes apache#6797 from viirya/fix_decimal and squashes the following commits:

8a447b1 [Liang-Chi Hsieh] Add unit test.
d67a5ea [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into fix_decimal
ab6d8af [Liang-Chi Hsieh] Fix setOrNull.

sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala

@@ -86,7 +86,7 @@ final class Decimal extends Ordered[Decimal] with Serializable {
       if (precision < 19) {
         return null  // Requested precision is too low to represent this value
       }
-      this.decimalVal = BigDecimal(longVal)
+      this.decimalVal = BigDecimal(unscaled)
       this.longVal = 0L
     } else {
       val p = POW_10(math.min(precision, MAX_LONG_DIGITS))

sql/catalyst/src/test/scala/org/apache/spark/sql/types/decimal/DecimalSuite.scala

@@ -156,4 +156,10 @@ class DecimalSuite extends SparkFunSuite with PrivateMethodTester {
     assert(Decimal(-100) % Decimal(3) === Decimal(-1))
     assert(Decimal(100) % Decimal(0) === null)
   }
+
+  test("set/setOrNull") {
+    assert(new Decimal().set(10L, 10, 0).toUnscaledLong === 10L)
+    assert(new Decimal().set(100L, 10, 0).toUnscaledLong === 100L)
+    assert(Decimal(Long.MaxValue, 100, 0).toUnscaledLong === Long.MaxValue)
+  }
 }