GODRIVER-298: support nil context to next method

Change-Id: Ia4005ae06ec7ae7e7525c5745a85e5ad77a058be

Author: tychoish

core/topology/cursor.go

@@ -79,6 +79,10 @@ func (c *cursor) ID() int64 {
 }
 
 func (c *cursor) Next(ctx context.Context) bool {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
 	c.current++
 	if c.current < c.batch.Len() {
 		return true

core/topology/cursor_test.go

@@ -0,0 +1,25 @@
+package topology
+
+import (
+	"testing"
+
+	"github.com/mongodb/mongo-go-driver/bson"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCursorNextDoesNotPanicIfContextisNil(t *testing.T) {
+	// all collection/cursor iterators should take contexts, but
+	// permit passing nils for contexts, which should not
+	// panic.
+	//
+	// While more through testing might be ideal this check
+	// prevents a regression of GODRIVER-298
+
+	c := cursor{batch: bson.NewArray(bson.VC.String("a"), bson.VC.String("b"))}
+
+	var iterNext bool
+	assert.NotPanics(t, func() {
+		iterNext = c.Next(nil)
+	})
+	assert.True(t, iterNext)
+}