add replace feature so that we can remock method result

[initlifeinc]

because sometime we need remock some method and return different mock result, and it seems expect feature only affect one by one, but sometimes we don't care or don't known how much times the method is invoked.

so use replace feature to replace existing mocked method and return a different result maybe a way to solved some special cases.

[erikdoe]

At a minimum I'd need a unit test to show and test this new feature. I'm not promising that I'll definitely merge it then, but it will help me make that decision.

[erikdoe]

Closing due to inactivity.