CLI: Promote "Host" header to `Request.Host`

# Summary

The Go HTTP client ignores `Host` header values.

To override the request `Host` header, you must set the [Request](https://golang.org/pkg/net/http/#Request) `Host` field.

## Use Cases

When communicating with a proxy or load balancer that does not do Host header rewriting, it may be required to override the default request `Host` header.

Go's HTTP request does not do this by default when passed a `Host` header value.

See https://github.com/golang/go/issues/29865

---

This is a super simple change and I'll raise a PR.

**Message from the maintainers**:

Love this enhancement proposal? Give it a 👍. We prioritize the proposals with the most 👍.

Trace the Go CLI's HTTP request construction and inspect how Host headers are handled, then find the existing request tests. Confirm the desired behavior against Go's Request.Host documentation and the linked issue. Done means a supplied Host header is promoted to Request.Host and the behavior is covered by tests.

Written by the indexing model from the issue text.