Fixing crash on nil object being mocked

I found that when a mock is created with a nil object OCMock will hard crash. This is because nil propagation hides the issue until a call to `objc_setAssociatedObject` where the class being nil causes a hard crash.

I've added explicit parameter asserts for the three mocks so that the issue can be caught immediately and, more importantly, will raise an exception that Xcode can catch rather than crashing the tests entirely.