Fix isolation of allow_message_expectations_on_nil
requested to merge github/fork/JonRowe/fix_isolation_of_allow_message_expectations_on_nil into master
Created by: JonRowe
In the process of fixing #276 I discovered that the test checking that allow_message_expectations_on_nil
was actually isolated per example wasn't being run.
So this makes that test run and then fixes the problem by moving the reset of the warning from MethodDouble to Proxy (which is where nil is being forcibly registered each time we enable it)