Migrate all specs to the newer expect syntax

Our specs are currently a mix of the older should syntax and the newer expect syntax. I wanted the excuse to give [Transpec](http://yujinakayama.me/transpec/) a try – as you can see, it seems to do a good job. I needed to make very a handful of subsequent fixes.

This relates to https://www.openproject.org/work_packages/4044 – and may assist with the upgrade to RSpec 3.