bug(httpExecutor): support changing request hostname

[titpetric]

When building a http request in Go, the Host header isn't honored. From the docs:

	// For client requests Host optionally overrides the Host
	// header to send. If empty, the Request.Write method uses
	// the value of URL.Host.
	Host string		Host string

I'd still expect the Host being set in the executor Headers, adding a check for it and setting req.Host respectively.