HTML Helper's startForm() doesn't pick up if current request is HTTPS

If a form is created on an HTTPS page with HTML.startForm(), the action is forced to HTTP instead of allowing the event.buildLink() call to detect the current page.

The fix is to not default the SSL attribute to anything. Check for its existence and only pass it along if it has been specified. Otherwise, allow event.buildLink() to default the URL protocol as necessary.