sse endpoint: hint to reverse-proxy not to buffer

Similarly to 210787a3, and as discussed in #88 and #130, nginx
will buffer chunked transfer-encoding unless proxy_buffering
is turned off or the X-Accel-Buffering header is set to no.

resolves #130

src/http.cpp

@@ -203,6 +203,8 @@ kj::Promise<void> Http::request(kj::HttpMethod method, kj::StringPtr url, const
     if(is_sse) {
         KJ_IF_MAYBE(s, fromUrl(url.cStr(), queryString)) {
             responseHeaders.set(kj::HttpHeaderId::CONTENT_TYPE, "text/event-stream");
+            // Disables nginx reverse-proxy's buffering. Necessary for streamed events.
+            responseHeaders.add("X-Accel-Buffering", "no");
             auto peer = kj::heap<WithSetRef<EventPeer>>(eventPeers);
             peer->scope = *s;
             std::string st = "data: " + laminar.getStatus(peer->scope) + "\n\n";