[SPARK-10980] [SQL] fix bug in create Decimal

The created decimal is wrong if using `Decimal(unscaled, precision, scale)` with unscaled > 1e18 and and precision > 18 and scale > 0.

This bug exists since the beginning.

Author: Davies Liu <davies@databricks.com>

Closes #9014 from davies/fix_decimal.

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

@@ -88,7 +88,7 @@ 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(unscaled)
+      this.decimalVal = BigDecimal(unscaled, scale)
       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

@@ -44,6 +44,7 @@ class DecimalSuite extends SparkFunSuite with PrivateMethodTester {
     checkDecimal(Decimal(170L, 4, 2), "1.70", 4, 2)
     checkDecimal(Decimal(17L, 24, 1), "1.7", 24, 1)
     checkDecimal(Decimal(1e17.toLong, 18, 0), 1e17.toLong.toString, 18, 0)
+    checkDecimal(Decimal(1000000000000000000L, 20, 2), "10000000000000000.00", 20, 2)
     checkDecimal(Decimal(Long.MaxValue), Long.MaxValue.toString, 20, 0)
     checkDecimal(Decimal(Long.MinValue), Long.MinValue.toString, 20, 0)
     intercept[IllegalArgumentException](Decimal(170L, 2, 1))