Add a GetStandardProperties trait.

src/parser/plain_text.rs

@@ -146,7 +146,7 @@ mod tests {
     use crate::context::GlobalSettings;
     use crate::context::List;
     use crate::parser::object_parser::detect_standard_set_object_sans_plain_text;
-    use crate::types::Source;
+    use crate::types::GetStandardProperties;
 
     #[test]
     fn plain_text_simple() {
@@ -159,6 +159,9 @@ mod tests {
         ))(&initial_context);
         let (remaining, result) = map(plain_text_matcher, Object::PlainText)(input).unwrap();
         assert_eq!(Into::<&str>::into(remaining), "");
-        assert_eq!(result.get_source(), Into::<&str>::into(input));
+        assert_eq!(
+            result.get_standard_properties().get_source(),
+            Into::<&str>::into(input)
+        );
     }
 }