net/http: sensitive headers incorrectly sent after cross-domain redirect

The HTTP client drops sensitive headers after following a cross-domain redirect.
For example, a request to a.com/ containing an Authorization header which is
redirected to b.com/ will not send that header to b.com.

In the event that the client received a subsequent same-domain redirect, however,
the sensitive headers would be restored. For example, a chain of redirects from
a.com/, to b.com/1, and finally to b.com/2 would incorrectly send the Authorization
header to b.com/2.

Thanks to Kyle Seely for reporting this issue.