Update RSpec examples to use new oneliner syntax

RSpec 3.0 will deprecate the monkey-patched `something.should ...` method, replacing it with `expect(something).to ...`. 
The new oneliner syntax uses `is_expected_to` instead of `should` in order to achieve consistent wording throughout the API.
See http://myronmars.to/n/dev-blog/2014/05/notable-changes-in-rspec-3#new_api_for_oneliners_