Send an message over the body stream.
All string data must be encoded using UTF-8 and multi-line data are handled properly by writing multiple 'data:' fields.
| event |
event name, or 'null' to omit the field |
| data |
event data |
| id |
event identifier, or 'null' to omit the field |
| retry |
retry, or 'null' to omit the field |
| Error |
errors are handled as warnings to avoid breaking the `text/event-stream` content |