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.

This particular issue affects our application and was the motivation for this patch. It's not obvious from stacktrace that NewRelic was the reason why the action stopped working on our staging environment while it worked in our development environment (in which case the newrelic gem is not loaded).

This can be specially dangerous if someone uses a separate server for staging (or doesn't use a staging server at all) and they don't use NewRelic in the staging server to save some money and then after deploying to production they realize the bug with no hint on what's causing it. Ideally adding the `newrelic_rpm` gem to some project shouldn't break the application in any circumstances. That's why I believe it's a good default to simply disable auto-instrumentation on live streaming requests until this could be implemented in a safer way.

Sorry but I couldn't set up a sample small application demonstrating the issue, but believe me: it does exist and it happens with our application. The proposed change makes it work for us even without disabling `auto_instrument`.

Related support ticket:
https://support.newrelic.com/requests/101179