Handle message newlines in event data

When message payload contains `\r`, `\n` or `\r\n` then by SSE spec they are canonicalized to `\n`.

**message**
```
1\n
2\r
3\r\n
4
```

**event**
```
data: 1\n
data: 2\n
data: 3\n
data: 4\n
\n
```

Therefore, we must inject `\ndata:` to replace `\r`, `\n` or `\r\n` in the message payload, and the event still starts with `data:` and ends with `\n\n` as usual.