goproxy buffers SSE (content-type: text/event-stream)

[waggledans]

The issue is well-described in coreos#5.

When using goproxy with SSE, the events are buffered, and a client just keeps waiting for data over an open TCP connection.

Scenario:

• set up an event source for server-side events, e.g. like in https://jasonbutz.info/2018/08/server-sent-events-with-node/
• curl it through goproxy (or use a proper SSE client in javascript using an EventSource)

Problem:

• goproxy buffers server-side events instead of proxying them as they are received