Add a work-around to let the 'markdown' filter render curly single quotes properly

[Redcarpet's issue 57](https://github.com/vmg/redcarpet/issues/57) outlines a bug with the `SmartyHTML` renderer, which causes single apostrophes to not be turned into curly single quotes as, e.g., `&rsquo;` There is an outstanding pull request to fix it, but until then, this is a work-around.

* The workaround is conditional on it being necessary.
* Tests are added in `test_helper.rb` so the tests will fail once it becomes unnecessary.
* The other monkey-patch for Liquid's `date` filter accepting `'now'` as input also now has a test assertion in `test_helper`.

This means that whenever Redcarpet's bug is fixed, not only will the workaround disable, but tests will catch it.