Fix ERB::Util::escape_json to stop it eating quotes

In 3-2-stable ERB::Util::escape_json silently eats quotes.

This is inconsistent with behavior in Rails 4, and it renders this method fairly useless. Certainly you cannot use it to escape JSON, which is the intended purpose of this method as far as I can tell from reading the method documentation and seeing that it differs in Rails 4.

See #13073 for related info.