Flush response more eagerly to support Server-Sent Events

Server-Sent Events are a [web standard][standard]. They require the headers to be sent before the body is ready. Also, the response stream should be flushed eagerly, so that events may be sent as they are written to the stream.

This pull request includes a function to copy and flush eagerly, and also flushes the response after headers are sent. It required an addition of a proxy method to the HTTPServletResponse testing mock.

[standard]: http://www.w3.org/TR/eventsource/