Switch to rspec expect syntax.

This stub syntax brings up deprecation warnings in rspec 3.0.  Spree
still operates on 2.14, but we use the auth helpers in an rspec 3
project.

The expect syntax is compatible with version of rspec newer than 2.11
introduced in Jun 2012.

Conflicts:
	core/lib/spree/testing_support/authorization_helpers.rb