HTTP client: Use Host field in URL if the Request Host is not present

[RonFed]

From the net/http documentation about the Request struct:

// For client requests, the URL's Host specifies the server to
// connect to, while the Request's Host field optionally
// specifies the Host header value to send in the HTTP
// request.
URL *url.URL

// For client requests, Host optionally overrides the Host
// header to send. If empty, the Request.Write method uses
// the value of URL.Host. Host may contain an international
// domain name.
Host string

Our current implementation can cause the host field to not be collected properly in some cases.
This can be reproduced using our e2e test by changing

resp, err := http.Get("http://user@localhost:8080/hello/42?query=true#fragment")

to

url, _ := url.Parse("http://user@localhost:8080/hello/42?query=true#fragment")
req := &http.Request{
  Method: http.MethodGet,
  URL:    url,
  }
req = req.WithContext(context.Background())
resp, err := http.DefaultClient.Do(req)

[damemi]

Thanks @RonFed, I actually wasn't sure if there was a difference between the URL Host and Request Host in #790 so glad you looked into it!