Readonly FieldBlock fails on submit

```
  File "iktomi/cms/stream_handlers.py", line 390, in edit_item_handler
    accepted = form.accept(request.POST)
  File "iktomi/forms/form.py", line 97, in accept
    field.set_raw_value(self.raw_data, field.from_python(value))
  File "iktomi/forms/fields.py", line 266, in set_raw_value
    .format(self.input_name, value)
AssertionError: To set raw value on '' need dict, got ''
```
