(#94) Add missing toMap fields

Author: lakscastro

lib/src/saf/document_file.dart

@@ -228,13 +228,16 @@ class DocumentFile {
 
   Map<String, dynamic> toMap() {
     return {
+      'id': id,
       'uri': '$uri',
       'parentUri': '$parentUri',
       'isDirectory': isDirectory,
       'isFile': isFile,
       'isVirtual': isVirtual,
       'name': name,
       'type': type,
+      'size': size,
+      'lastModified': lastModified?.millisecondsSinceEpoch,
     };
   }