Make extension methods more testable

[oliverheywood451]

This stack overflow describes the situation pretty well.
https://stackoverflow.com/questions/2295960/mocking-extension-methods-with-moq

In our case the extension methods are ListAll() and ListByIDs()

Brought up by Crhistian - when you are writing tests for functions that use a ListAll function, mocking the IOrderCloudClient doesn't cut it, because it doesn't have that method.