caption is corrected for files

configs.go

@@ -349,6 +349,7 @@ func (config AudioConfig) method() string {
 // DocumentConfig contains information about a SendDocument request.
 type DocumentConfig struct {
 	BaseFile
+	Caption  string
 }
 
 // values returns a url.Values representation of DocumentConfig.
@@ -360,6 +361,10 @@ func (config DocumentConfig) values() (url.Values, error) {
 
 	v.Add(config.name(), config.FileID)
 
+	if config.Caption != "" {
+		v.Add("caption", config.Caption)
+	}
+
 	return v, nil
 }