Fix setting of nested non-overridable properties via Mock.Of

[stakx]

Fixes #1039.

[stakx]

Perhaps we could go one step further here and add support for indexers by passing through the index arguments instead of hard-coding null as the last argument.

[stakx]

Leaving this unresolved for now. Supporting indexers for non-overridable properties would only work partly anyway, e.g. you could only use fixed values (no matchers) for the index arguments. Not sure if it's worth doing the work to support this scenario as far as is possible.

[stakx]

Note to self: It may be worth reconsidering whether InvocationShape should do any validation at all with regards to method overridability.

The logic inside this type should work just as well for non-overridable methods, it's external parties that add the "method must be overridable (and visible to the proxy factory)" requirement.

It might therefore be better to extract validation from InvocationShape, and move it somewhere else. (I was going to say [Castle]ProxyFactory, but that wouldn't be totally right, since it isn't just the proxy factories that require methods to be overridable, but lots of other parts of Moq, too.)

Each party that currently builds up InvocationShapes would then have to explicitly validate them as needed. This way, the allowNonOverridable… parameters being added in this PR could be dropped again.