FIX content-type to be compatible with HTML5 EventSource expectations

[xnopre]

Replace HttpResponse with StreamingHttpResponse for event-stream content

[xnopre]

This package is simple and usefull, thanks ! But actually, we have a problem.

With this Javascript code :

const source = new EventSource('sse/');
source.onmessage = function(event) {
    ...
};

we have this error :

EventSource’s response has a MIME type (“text/html”) that is not “text/event-stream”. Aborting the connection.

This seems to be fixed by @ivencity but there is not PR. So I have forked his modifications to create this PR. I hope this can be merge as soon as possible.

Thanks