operator-surround-replace kills indent in visual mode

Let `.vimrc` contain

```
nnoremap <script> <SID>(hvgu) ^vg_
nmap <silent> css <SID>(hvgu)<Plug>(operator-surround-replace)
```

and let `test.vim` consist of the single line

```
  (nmap csg)
```

Expected: hitting `cssB` gives

```
  {nmap csg}
```

Result:

```
{nmap csg}
```
