don't close the top-level addr

See https://github.com/ipfs/go-ipfs-http-client/pull/10/files#r269268326

tests/unixfs.go

@@ -834,6 +834,7 @@ type closeTestD struct {
 }
 
 func (f *closeTestD) Close() error {
+	f.t.Helper()
 	if f.closed {
 		f.t.Fatal("already closed")
 	}
@@ -874,8 +875,6 @@ func (tp *provider) TestAddCloses(t *testing.T) {
 		t.Error(err)
 	}
 
-	d0.Close() // Adder doesn't close top-level file
-
 	for i, n := range []*closeTestF{n1, n2, n4} {
 		if !n.closed {
 			t.Errorf("file %d not closed!", i)