bugfix: TestFutureSetStateRaceCondition data race

A use-case from our code: a response per AppendPush(). It looks like
it's enough to change the test.

Part of #218

Author: oleg-jukovec

future_test.go

@@ -237,13 +237,13 @@ func TestFutureGetIteratorError(t *testing.T) {
 func TestFutureSetStateRaceCondition(t *testing.T) {
 	err := errors.New("any error")
 	resp := &Response{}
-	respAppend := &Response{}
 
 	for i := 0; i < 1000; i++ {
 		fut := NewFuture()
 		for j := 0; j < 9; j++ {
 			go func(opt int) {
 				if opt%3 == 0 {
+					respAppend := &Response{}
 					fut.AppendPush(respAppend)
 				} else if opt%3 == 1 {
 					fut.SetError(err)