Fixed behaviour of property injection for private properties in class hierarchies

It was not possible to use property injection for private fields in class hierarchies (not sure about protected maybe the same issue appeared there as well, the issue https://github.com/watoki/factory/issues/1 references this behaviour).

I only added a test for private fields since the problem appeared to me in this context. We might want to change the test (or add an additional one) to prove that property injection works as well for protected and public fields if they were inherited.