Persistent config 'in' operator

[Stuart Bishop]

If user provided config is loaded from state, those items are not found using the 'in' operator.

I believe the reason for this is http://bugs.python.org/issue10977 , which in summary means 'don't override builltin classes'. Methods you override will not be invoked by code using the C API, instead using the builtin implementation and causing chaos and destruction.

There are tests for the 'in' operator, but they did not actually catch this because the 'main' dictionary was mirroring the contents of the _prev_dict attribute