add spec for Item#to_json

To write spec easily, use as_json instead of to_json.
fix String#scrub_html

Because feedzirra(lib/feedzirra/core_ext/string.rb) introduce String#sanitize,
String#scrub_html cannot call ActionView::Helpers::SanitizeHelper#sanitize.
Merge pull request #96 from mzp/fix_relative_date

Fix relative date