Update parse_affiliated_keywords for handling optional pairs.

src/types/affiliated_keyword.rs

@@ -6,7 +6,11 @@ use super::Object;
 pub enum AffiliatedKeywordValue<'s> {
     SingleString(&'s str),
     ListOfStrings(Vec<&'s str>),
-    ListOfListsOfObjects(Vec<(Option<Vec<Object<'s>>>, Vec<Object<'s>>)>),
+    OptionalPair {
+        optval: Option<&'s str>,
+        val: &'s str,
+    },
+    ObjectTree(Vec<(Option<Vec<Object<'s>>>, Vec<Object<'s>>)>),
 }
 
 #[derive(Debug)]