net/http: 307 and 308 status codes should not change method on redirect

[subbu05]

I have gone through few golang issues on HTTP redirect.
I have referred to HTTP RFC https://tools.ietf.org/html/rfc7238 and conveys that client need not change the PUT/POST method when 307 or 308 response is seen in the request.

Note: This status code is similar to 301 (Moved Permanently)
([RFC7231], Section 6.4.2), except that it does not allow changing
the request method from POST to GET.

But with golang 1.7 I see that net http client is using HTTP GET when 307 or 308 response is seen. This violates the http rfc which says HTTP method will not be changed for 307 or 308 http response.

[quentinmit]

/cc @bradfitz

[bradfitz]

Dup of #10767