Use Capybara::Helpers.normalize_whitespace in filter_text()

Text that can be obtained in `visible_text()` and `all_text()` is
not stripped unicode whitespace (e.g. \u3000 "IDEOGRAPHIC SPACE").

`capybara/selenium` is able to delete unicode whitespace
[using `Capybara::Helpers.normalize_whitespace`](https://github.com/jnicklas/capybara/blob/master/lib/capybara/selenium/node.rb#L9)

So, I'd like to unify to using `Capybara::Helpers.normalize_whitespace`.