Replace nil context with context.Background in Client Disconnect.

GODRIVER-767

Change-Id: I9f63d169a1bdf8ef2e7c0d355f29a83cc96f0661

Author: Divjot Arora

mongo/client.go

@@ -100,6 +100,10 @@ func (c *Client) Connect(ctx context.Context) error {
 // or write operations. If this method returns with no errors, all connections
 // associated with this Client have been closed.
 func (c *Client) Disconnect(ctx context.Context) error {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
 	c.endSessions(ctx)
 	return replaceTopologyErr(c.topology.Disconnect(ctx))
 }

mongo/client_internal_test.go

@@ -538,3 +538,13 @@ func TestClient_Ping_InvalidHost(t *testing.T) {
 	err = c.Ping(ctx, nil)
 	require.NotNil(t, err)
 }
+
+func TestClient_Disconnect_NilContext(t *testing.T) {
+	cs := testutil.ConnString(t)
+	c, err := NewClient(cs.String())
+	require.NoError(t, err)
+	err = c.Connect(nil)
+	require.NoError(t, err)
+	err = c.Disconnect(nil)
+	require.NoError(t, err)
+}