Set req.Host for V4 sigs only

This one shouldn't break other code since this method is used exclusively by changes introduced in #223

The reason for setting `req.Host` is this line:
https://github.com/crowdmob/goamz/blob/master/aws/sign.go#L150

Maybe it could use `req.URL.Host` if `req.Host == ""` there, but that could affect existing uses of the `V4Signer`.