Set a default `expects` parameter to be 2xx

[chrisjhoughton]

The vast majority of the time (like, always) this will be what's required. We should set it by default on the global level so we don't even have to worry about expects handling unless we're specifically trying to catch errors.

[chrisjhoughton]

Dependent on https://github.com/trayio/threadneedle/issues/8 first