fix(bulk): use original indexes as map for current op index

The fix for bulk introduced in v3.4.0 did not take into account
the index of the operation result when referring to the original
stored index map.

NODE-2383

lib/bulk/common.js

@@ -475,7 +475,7 @@ function mergeBatchResults(batch, bulkResult, err, result) {
   if (Array.isArray(result.writeErrors)) {
     for (let i = 0; i < result.writeErrors.length; i++) {
       const writeError = {
-        index: batch.originalIndexes[i],
+        index: batch.originalIndexes[result.writeErrors[i].index],
         code: result.writeErrors[i].code,
         errmsg: result.writeErrors[i].errmsg,
         op: batch.operations[result.writeErrors[i].index]

test/functional/bulk.test.js

@@ -1660,4 +1660,41 @@ describe('Bulk', function() {
         );
     });
   });
+
+  it('should preserve order of operation index in unordered bulk operation', function() {
+    const client = this.configuration.newClient();
+    return client.connect().then(() => {
+      this.defer(() => client.close());
+
+      const coll = client.db().collection('bulk_op_ordering_test');
+      function ignoreNsNotFound(err) {
+        if (!err.message.match(/ns not found/)) throw err;
+      }
+
+      return coll
+        .drop()
+        .catch(ignoreNsNotFound)
+        .then(() => {
+          const batch = coll.initializeUnorderedBulkOp();
+          batch.insert({ _id: 1, a: 0 });
+          batch.insert({ _id: 1, a: 0 });
+          batch.insert({ _id: 2, a: 0 });
+          batch.insert({ _id: 2, a: 0 });
+          return batch.execute();
+        })
+        .then(
+          () => {
+            throw new Error('expected a bulk error');
+          },
+          err => {
+            expect(err)
+              .to.have.property('writeErrors')
+              .with.length(2);
+
+            expect(err).to.have.nested.property('writeErrors[0].err.index', 1);
+            expect(err).to.have.nested.property('writeErrors[1].err.index', 3);
+          }
+        );
+    });
+  });
 });