Fix token basic auth header issue

When requesting a token, the basic auth header is always being set even
if there is no username value. This patch corrects this and does not set
the basic auth header if the username is empty.

Also fixes an issue where pulling all tags from a v2 registry succeeds
when the image does not actually exist on the registry.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

docs/session_v2.go

@@ -128,6 +128,8 @@ func (r *Session) HeadV2ImageBlob(ep *Endpoint, imageName, sumType, sum string,
 	case res.StatusCode >= 200 && res.StatusCode < 400:
 		// return something indicating no push needed
 		return true, nil
+	case res.StatusCode == 401:
+		return false, errLoginRequired
 	case res.StatusCode == 404:
 		// return something indicating blob push needed
 		return false, nil

docs/token.go

@@ -51,10 +51,12 @@ func getToken(username, password string, params map[string]string, registryEndpo
 		reqParams.Add("scope", scopeField)
 	}
 
+	if username != "" {
 		reqParams.Add("account", username)
+		req.SetBasicAuth(username, password)
+	}
 
 	req.URL.RawQuery = reqParams.Encode()
-	req.SetBasicAuth(username, password)
 
 	resp, err := client.Do(req)
 	if err != nil {