net/http: handle 3xx redirects properly

net/http: handle 3xx redirects properly

Provides redirection support for 307, 308 server statuses.
Provides redirection support for DELETE method.

Updates old tests that assumed all redirects were treated
the way 301, 302 and 303 are processed.

Fixes #9348
Fixes #10767
Fixes #13994

Change-Id: Iffa8dbe0ff28a1afa8da59869290ec805b1dd2c4
Reviewed-on: https://go-review.googlesource.com/29852
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>