fix: adjust "Host" via http helper

[estahn]

I would like to do this:

headers := map[string]string{"Host" : "gloo-running-7xhydj.foobar"}
code, result := http_helper.HTTPDo(t, "GET", "http://randomelb.elb.ap-southeast-2.amazonaws.com/", nil, headers, nil)

[estahn]

@robmorgan Can you have a look at this one?

[yorinasub17]

LGTM! Thanks for the contribution and apologies for the delay in reviewing this! I'll kick off a regression build and if it passes, we can merge this in.

[yorinasub17]

The build failed, but for reasons unrelated to this change so I am comfortable merging this in. Thanks again for the contribution here!