Requests without "Accept: text/event-stream" header are ignored

Hi !

First of all, thanks for this neat & useful library !

I'm seeing here :

Lib.AspNetCore.ServerSentEvents/Lib.AspNetCore.ServerSentEvents/ServerSentEventsMiddleware.cs

Line 55 in 0b94dc1

    
           if (context.Request.Headers[Constants.ACCEPT_HTTP_HEADER] == Constants.SSE_CONTENT_TYPE)

that if the incoming request does not have the Accept: text/event-stream header then the middleware is not processing it.

I am not sure if this request header is mandatory, here https://html.spec.whatwg.org/multipage/server-sent-events.html#the-eventsource-interface , in the paragaph

The EventSource(url, eventSourceInitDict) constructor, when invoked, must run these steps:

the item 10 is then:

User agents may set Accept/text/event-stream in request's header list.

may not must ...

Could you consider having this behavior configurable / optional ?
What do you think ? Am I interpreting something wrong ?

Again, thanks for sharing this lib !

I believe your interpretation to be correct. In fact I think the statement below from different version of spec describes it the best.

For HTTP connections, the Accept header may be included; if included, it must contain only formats of event framing that are supported by the user agent (one of which must be text/event-stream).

So if Accept requet header is present, it should contain text/event-stream, but the Accept request header doesn't have to be present.

I'll treat it as a low priority bug.

I'm sorry, I'm not able to run the test in my environment, so I cannot propose a PR ...

Following your comment, I would process the request:

if the Accept header is there with "text/event-stream" value
or if the Accept is not there

My proposed approach would then be to replace the current if block by the following one :

if ( (  context.Request.Headers.ContainsKey(Constants.ACCEPT_HTTP_HEADER)
        &&
        context.Request.Headers[Constants.ACCEPT_HTTP_HEADER] == Constants.SSE_CONTENT_TYPE ) 
    ||
    ! context.Request.Headers.ContainsKey(Constants.ACCEPT_HTTP_HEADER)
    )