Fix: (heading) Only match against heading text

It should not match against e.g. tags strings or to-do keywords.  Note
that the (regexp) predicate still matches against those, because it
searches entire entries, from heading stars to the next entry.

README.org

@@ -488,6 +488,9 @@ Expands into a call to ~org-ql-select~ with the same arguments.  For convenience
 +  Predicate =heading= now accepts multiple regexps, which are matched with boolean =AND=.
 +  Predicate =regexp= now matches its regexp arguments with boolean =AND=.
 
+*Fixed*
++  Predicate =heading= now matches only against heading text, i.e. not including tags at the end of the line, to-do keyword, etc.
+
 ** 0.2.1
 
 *Fixed*

org-ql.el

@@ -448,12 +448,14 @@ replace the clause with a preamble."
                                  nil)
 
                                 ;; Heading text.
+                                ;; MAYBE: Adjust regexp to avoid matching in tag list.
                                 (`(heading ,regexp)
-                                 ;; Only one regexp: match with preamble only.
+                                 ;; Only one regexp: match with preamble, then let predicate confirm (because
+                                 ;; the match could be in e.g. the tags rather than the heading text).
                                  (setq org-ql-preamble (rx-to-string `(seq bol (1+ "*") (1+ blank) (0+ nonl)
                                                                            ,regexp)
                                                                      'no-group))
-                                 nil)
+                                 element)
                                 (`(heading . ,regexps)
                                  ;; Multiple regexps: use preamble to match against first
                                  ;; regexp, then let the predicate match the rest.