Subject blocks called multiple times for a single example

Created by: lackac

I ran into an issue where my explicit subject blocks were called multiple times. This caused my expectations set up with mocks to fail because through the subject block a stubbed method was also called multiple times.

I investigated the cause of this and it became clear that the subject block was only called multiple times if its return value was either nil or false. Caching the return value of the subject block should handle these cases as well.

Attached is a pull request with a scenario and a fix for this issue.