Method to_hash_map

```rust use orgize::{Org, ast::PropertyDrawer}; let org = Org::parse("* Heading\n:PROPERTIES:\n:CUSTOM_ID: someid\n:CUSTOM_ID: id\n:END:"); let drawer = org.first_node:: ().unwrap(); let map = drawer.to_hash_map(); assert_eq!(map.len(), 1); assert_eq!(map.get("CUSTOM_ID").unwrap(), "id"); ```

(&self)