[fix] exclude Host header from load fetch requests made on server

[Conduitry]

Having child requests made via fetch from load during SSR include the Host header is bad. At the very least, it causes SSL certificate issues. (I'm not sure whether it causes other issues in shared-IP / dynamic host environments - I don't know whether the value in the headers or in the URL takes precedence in node-fetch.)

This skips over the Host header as well when copying the headers into the outgoing request. I looked at the existing test added in #3631, but I wasn't sure how to extend it.

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