Add dummy values for new fields for plaintext.

src/types/object.rs

@@ -1014,11 +1014,13 @@ impl<'s> StandardProperties<'s> for PlainText<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        todo!()
+        // This field does not actually exist in emacs for plaintext
+        Some(self.source)
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        todo!()
+        // This field does not actually exist in emacs for plaintext
+        0
     }
 }