Cannot send Server-Sent Events without data using WebMvc.fn SseBuilder

[bclozel]

The Server-Sent Events specification does not require events to have a data: field in them. The current SeeBuilder spec in WebMvc.fn enforces this because all terminal methods require a data: field.

We should add a send() operation to send all buffered id, event and comment as is, without adding any data field. Because the spec doesn't require data: fields, we are considering this as a bug.

This will be useful for implementing the GraphQL over SSE transport in Spring for GraphQL.