No remote request flag is set when sending such frame with interface socketcand

### Describe the bug
When sending a frame like
```python
bus = can.interface.Bus(interface='socketcand', channel='can0', host='the.ipa.ddr.ess', port=29536)
...
can_msg = can.Message(is_extended_id = False, arbitration_id = address, is_remote_frame = True)
self.bus.send(can_msg)
```
the terminal message print is:
```
Timestamp: 0.000000 ID: 403 S Rx R DL: 0
```
but on the bus be get (using candump):
```
can0 403 [0] ''
```
kinda empty frames.

When using the direct can0 interface (no socketcand) I get correctly:
```
can0 403 [0] remote request
```

### To Reproduce
Using python_can==4.6.1 on a Mac with Python 3.9.6.
socketcand Version 0.6.1 is running on a Debian 13.

### Expected behavior
Seeing correct remote request frames

Start at can.interface.Bus with interface='socketcand' and trace self.bus.send(can_msg) when can.Message sets is_remote_frame=True. Compare the socketcand path with the direct can0 path and use candump to verify the result. Done means the transmitted frame is reported as a remote request rather than an empty data frame.

由索引模型根据 Issue 内容生成。