Use RSpec's expect syntax

The reasons to do so include:

* Style: it highlights the verification phase of the Four-Phase Test
  well by starting each line with `expect`, wrapping the outcome in the
  parentheses.
* Consistency across RSpec specs.
* Consistency with Jasmine.
* Avoids weird, confusing errors when testing delegate/proxy objects.
* Avoids Ruby warning with `should ==`.