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`.
Add specs for Capybara::Poltergeist::Node#filter_text
Fix specs for Capybara::Poltergeist::Node#filter_text
Merge pull request #391 from gongo/use_capybara_whitespace_stripping

Use Capybara::Helpers.normalize_whitespace in filter_text()