Fix Transport-Encoding: chunked template streaming

[sfcgeorge]

Using chunked template streaming in Rails via render stream: true doesn't work with Bullet. Much like with the server sent events issue when using (ActionController::Live) you can't mess with the stream this way.

The error using curl is "Illegal or missing hexadecimal sequence in chunked-encoding".

I've added an easy fix, checking the header.

I've also added tests for the existing sse? method which I amended.