HEAD requests should in my opinion have the ability to follow redirects
like the implementation of GET requests does. My use case is polling
several thousand severs to check if they respond with 200 status codes.
Using GET requests is neither efficient in running time of the task nor
for bandwidth consumption.

This suggested patch changes the return signature of http.Head() to match
that of http.Get(), providing the final URL in a redirect chain.

`curl -IL http://google.com` follows redirects with HEAD requests just fine.

Fixes #1806.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4517058