Fix heading post-blank.

org_mode_samples/sections_and_headings/heading_with_subheading_post_blank.org

@@ -0,0 +1,5 @@
+* foo
+
+** bar
+
+* baz

src/types/document.rs

@@ -103,16 +103,11 @@ impl<'s> StandardProperties<'s> for Heading<'s> {
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        self.children
+        self.post_blank
-            .last()
+            .map(|text| text.lines().count())
-            .map(|child| child.get_post_blank())
+            .unwrap_or(0)
-            .unwrap_or(
+            .try_into()
-                self.post_blank
+            .expect("Too much post-blank to fit into a PostBlank.")
-                    .map(|text| text.lines().count())
-                    .unwrap_or(0)
-                    .try_into()
-                    .expect("Too much post-blank to fit into a PostBlank."),
-            )
     }
 }