Refused to set unsafe header "connection"

I've noticed this error in Safari (9.1.2) and found in domain-task the following:

        if (!init.headers['Connection']) {
            init.headers['Connection'] = 'keep-alive';
        }

I see Connection listed here as a forbidden header name in the [fetch spec](https://fetch.spec.whatwg.org/#forbidden-header-name).

Can this be wrapped in a !isBrowser condition?