Disable auto-instrumentation for streamed requests

`autoinstrument_source` will force a read on the response (`gather_source`)
 which is not desired for live streaming requests. Worse than that, a working
 application may stop working once NewRelic is enabled because the gem may
 yield a `commit!` (on source reading) before the last header has been set
 which will raise an exception when another middleware tries to set some
 header due to the cookies hash being frozen by Rails after the first `commit!`.

While setting auto_instrument to false in newrelic.yml works, I believe a
better approach would be to disable auto-instrumentation for streamed responses.