Add default serveraddress value in remote API /auth

This fix tries to address the issue in #22244 where the remote
API `/auth` will not set the default value of `serveraddress`
if not provided. This behavior happens after only in 1.11.0
and is a regression as in 1.10.3 `serveraddress` will be assigned
with `IndexServer` if no value is provided.

The default value `IndexServer` is assigned to `serveraddress` if
no value provided in this fix.

An integration test `TestAuthApi` has been added to cover this change

This fix fixes #22244.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/service.go

@@ -37,6 +37,9 @@ func (s *Service) ServiceConfig() *registrytypes.ServiceConfig {
 // It can be used to verify the validity of a client's credentials.
 func (s *Service) Auth(authConfig *types.AuthConfig, userAgent string) (status, token string, err error) {
 	serverAddress := authConfig.ServerAddress
+	if serverAddress == "" {
+		serverAddress = IndexServer
+	}
 	if !strings.HasPrefix(serverAddress, "https://") && !strings.HasPrefix(serverAddress, "http://") {
 		serverAddress = "https://" + serverAddress
 	}