registry.Ping() may need to check response.statuscode

[aiyijing]

when use http protocol to connect https registry,it will return 302 response.
here would better check statuscode and raise Error

docker-registry-client/registry/registry.go

Lines 110 to 118 in afc9e1a

	func (r *Registry) Ping() error {
	url := r.url("/v2/")
	r.Logf("registry.ping url=%s", url)
	resp, err := r.Client.Get(url)
	if resp != nil {
	defer resp.Body.Close()
	}
	return err
	}