net/http: regression in client redirect logic

[dsnet]

Commit 7db996e alters the behavior of redirects with the HEAD method unnecessarily.

The logic in Go1.7 had the following properties:

• It automatically handled redirects for GET, HEAD, PUT, POST methods only.
• However, in the situations where the method was PUT or POST, it would convert it to be the GET method.
• Thus, all methods were redirected as GET except HEAD, which continued to be redirected as HEAD.

The logic in Go1.8 has the following properties:

• For 301, 302, and 303 redirects, the redirect method would always be set to GET.

For backwards compatibility reasons, we should keep redirecting HEAD as HEAD. There is no harm in preserving this old behavior since the RFC doesn't specify that methods must be set to GET for 301, 302, and 303 redirect.

Reproducer: https://play.golang.org/p/kKuGNRjHV9

\cc @bradfitz @odeke-em

[gopherbot]

CL https://golang.org/cl/34981 mentions this issue.