Add a post blank implementation to document.

src/types/document.rs

@@ -66,7 +66,10 @@ impl<'s> StandardProperties<'s> for Document<'s> {
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        todo!()
+        self.into_iter()
+            .last()
+            .map(|child| child.get_post_blank())
+            .unwrap_or(0)
     }
 }