fix: enable spell checking, which broke in upgrade to ch66

Chromium commit [03563dd163][1] changed the way that the
spellcheck-enabled status was checked, defaulting to false.

Added the first (!) test for spellchecking, too.

Fixes #13608.

[1]: https://chromium.googlesource.com/chromium/src/+/03563dd1635a028d286beaf8b4f3d637cc4a8830

atom/renderer/api/atom_api_spell_check_client.cc

@@ -115,6 +115,10 @@ void SpellCheckClient::RequestCheckingOfText(
                      base::Owned(pending_request_param_.release())));
 }
 
+bool SpellCheckClient::IsSpellCheckingEnabled() const {
+  return true;
+}
+
 void SpellCheckClient::ShowSpellingUI(bool show) {}
 
 bool SpellCheckClient::IsShowingSpellingUI() {