Use to_s when coercing IRubyObject to a RubyString.

Beyond just symbols, it seems a lot of things use #to_s instead of #to_str. Using `asString()` attempts to use `to_s` under the covers, while `convertToString()` uses `to_str`.

It appears EscapeUtils calls `to_s` before passing it off to Houdini (https://github.com/brianmario/escape_utils/blob/master/lib/escape_utils/html_safety.rb), but seeing as Walters aliases most of the methods, I just dropped it into the extension.