Resolve message endpoint path using request context path #400
Description
When HttpServletSseServerTransportProvider handling the GET request to establish a new SSE connection, the message endpoint is advertised using the immutable field "baseUrl" that's set in the constructor:
// Send initial endpoint event this.sendEvent(writer, ENDPOINT_EVENT_TYPE, this.baseUrl + this.messageEndpoint + "?sessionId=" + sessionId);
This will not work if clients use a different base URL to access this servlet (e.g.: using reverse proxies). The servlet implementation should allow the request context path to be used to produce the endpoint path appropriate for that request:
// Send initial endpoint event this.sendEvent(writer, ENDPOINT_EVENT_TYPE, request.getContextPath() + this.messageEndpoint + "?sessionId=" + sessionId);