When following a redirect (e.g. 302), doFollowingRedirects will call URL.Parse() on the
value of the Location header, then use URL.String() from the resulting object as the new
URL. URL.String(URL.Parse(x)) doesn't round-trip. In particular, %2F gets flattened to
"/" but never converted back. This caveat is noted here:
http://golang.org/pkg/net/url/#URL.

As a result, a server can't actually redirect a Go client to a URL with %2F in it. This
disagrees with the behavior of at least Chrome and curl.

I expect the best option here is to make URL.Parse / URL.String round-trip, since the
%2F exception is weird and surprising. At the very least, net/http should keep the
verbatim URL when redirecting.