Fix: (heading) Don't match strings as regexps

Reported-by: Alex Popescu <https://github.com/al3xandru>

org-ql.info

@@ -1076,7 +1076,12 @@ File: README.info,  Node: 085-pre,  Next: 084,  Up: Changelog
 5.1 0.8.5-pre
 =============
 
-Nothing new yet.
+*Fixes*
+   • Predicate ‘heading’ incorrectly matched strings as regular
+     expressions, sometimes returning incorrect results.  (See
+     discussion (https://github.com/alphapapa/org-ql/discussions/410).
+     Thanks to Alex Popescu (https://github.com/al3xandru) for
+     reporting.)
 
 
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