refactored spec expectations using `should` over to `expect`

A quick refactoring to remove the RSpec 3.x deprecation warnings in case there isn't plans to continue using the `should` syntax.

[https://github.com/rspec/rspec-expectations/commit/dc8406b37eda6f58f9426dd8eec52fa7d7d186bc](https://github.com/rspec/rspec-expectations/commit/dc8406b37eda6f58f9426dd8eec52fa7d7d186bc)