Fix KVO observer leakage in SSArrayDataSource

This fixes the [observer leakage][n] without introducing a [possible
retain cycle][rc]. I'm not sure why `weak` doesn't work for us.

We still need handle the situation where `target` could get unexpectedly
get deallocated since it's no longer automatically nil'd out.

[n]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39612129
[rc]: https://github.com/splinesoft/SSDataSources/pull/29#issuecomment-39646222