Can't set Host header

[tsenart]

Can't set Host header

[tsenart]

We can't set the Host header differently that what the request URL is because that wouldn't be compliant to the HTTP spec. You can check the code that prevents this here:
http://golang.org/src/pkg/net/http/request.go#L61
http://golang.org/src/pkg/net/http/request.go#L330
http://golang.org/src/pkg/net/http/request.go#L385

I agree that this is very useful in certain situations. Please open a PR with your change set and test it if you want brevity on this issue. Otherwise I can do it too, but I can't promise it will be very soon.

The copy of the headers is necessary because the net/http pkg mutates the headers underneath for various reasons (like setting auth headers from the url info, the host header, etc). You can check out this old issue for further reference: #30