[SPARK-10184] [CORE] Optimization for bounds determination in RangePartitioner

JIRA Issue: https://issues.apache.org/jira/browse/SPARK-10184

Change `cumWeight > target` to `cumWeight >= target` in `RangePartitioner.determineBounds` method to make the output partitions more balanced.

Author: ihainan <ihainan72@gmail.com>

Closes #8397 from ihainan/opt_for_rangepartitioner.

core/src/main/scala/org/apache/spark/Partitioner.scala

@@ -291,7 +291,7 @@ private[spark] object RangePartitioner {
     while ((i < numCandidates) && (j < partitions - 1)) {
       val (key, weight) = ordered(i)
       cumWeight += weight
-      if (cumWeight > target) {
+      if (cumWeight >= target) {
         // Skip duplicate values.
         if (previousBound.isEmpty || ordering.gt(key, previousBound.get)) {
           bounds += key