Add Accept: text/event-stream header

[williamkapke]

Simple enough change: Tell the server what kind of response we are looking for.

[williamkapke]

Additional info: http://www.w3.org/TR/eventsource/#the-eventsource-interface

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, as described below).