Change interface check to HTMLDocument again (#518)

Author: jonathanKingston

src/content-scope-features.js

@@ -8,7 +8,8 @@ import platformFeatures from 'ddg:platformFeatures'
 function shouldRun () {
     // don't inject into non-HTML documents (such as XML documents)
     // but do inject into XHTML documents
-    if (document instanceof Document === false && (
+    // Should check HTMLDocument as Document is an alias for XMLDocument also.
+    if (document instanceof HTMLDocument === false && (
         document instanceof XMLDocument === false ||
         document.createElement('div') instanceof HTMLDivElement === false
     )) {