fix: HTMLTagList to include headings

Author: andreypopp

src/HTMLTagList.js

@@ -30,6 +30,12 @@ let HTMLTagList: {[name: string]: ?boolean} = {
   'center': true,
   'cite': true,
   'code': true,
+  'h1': true,
+  'h2': true,
+  'h3': true,
+  'h4': true,
+  'h5': true,
+  'h6': true,
   'col': true,
   'colgroup': true,
   'command': true,