Config-file: remove User-Agent from config.json when saving

The config.json allows for setting custom HTTP headers, but given that
User-Agent is not customizable, we should remove it from the config before saving;

Before this change;

    $ cat ~/.docker/config.json
    {
        "auths": {
            "https://index.docker.io/v1/": {
                "auth": "<base64 auth>"
            }
        },
        "HttpHeaders": {
            "User-Agent": "Docker-Client/19.03.12 (linux)"
        }
    }

    $ docker logout
    {
        "auths": {},
        "HttpHeaders": {
            "User-Agent": "Docker-Client/19.03.12 (linux)"
        }
    }

After this change:

    $ cat ~/.docker/config.json
    {
        "auths": {
            "https://index.docker.io/v1/": {
                "auth": "<base64 auth>"
            }
        },
        "HttpHeaders": {
            "User-Agent": "Docker-Client/19.03.12 (linux)"
        }
    }

    $ docker logout
    Removing login credentials for https://index.docker.io/v1/

    $ cat ~/.docker/config.json
    {
        "auths": {}
    }

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/config/configfile/file.go

@@ -169,6 +169,13 @@ func (configFile *ConfigFile) SaveToWriter(writer io.Writer) error {
 	configFile.AuthConfigs = tmpAuthConfigs
 	defer func() { configFile.AuthConfigs = saveAuthConfigs }()
 
+	// User-Agent header is automatically set, and should not be stored in the configuration
+	for v := range configFile.HTTPHeaders {
+		if strings.EqualFold(v, "User-Agent") {
+			delete(configFile.HTTPHeaders, v)
+		}
+	}
+
 	data, err := json.MarshalIndent(configFile, "", "\t")
 	if err != nil {
 		return err

cli/config/configfile/file_test.go

@@ -466,7 +466,27 @@ func TestSave(t *testing.T) {
 	assert.NilError(t, err)
 	cfg, err := ioutil.ReadFile("test-save")
 	assert.NilError(t, err)
-	assert.Check(t, is.Equal(string(cfg), "{\n	\"auths\": {}\n}"))
+	assert.Equal(t, string(cfg), `{
+	"auths": {}
+}`)
+}
+
+func TestSaveCustomHTTPHeaders(t *testing.T) {
+	configFile := New(t.Name())
+	defer os.Remove(t.Name())
+	configFile.HTTPHeaders["CUSTOM-HEADER"] = "custom-value"
+	configFile.HTTPHeaders["User-Agent"] = "user-agent 1"
+	configFile.HTTPHeaders["user-agent"] = "user-agent 2"
+	err := configFile.Save()
+	assert.NilError(t, err)
+	cfg, err := ioutil.ReadFile(t.Name())
+	assert.NilError(t, err)
+	assert.Equal(t, string(cfg), `{
+	"auths": {},
+	"HttpHeaders": {
+		"CUSTOM-HEADER": "custom-value"
+	}
+}`)
 }
 
 func TestSaveWithSymlink(t *testing.T) {