Fix chunckedLoading LatLngBounds.intersects()

[boldtrn]

This PR fixes #743. We use Leaflet.markercluser with chunckedLoading in production and have seen issues similar to the ones that are reported in #743.

We applied this fix over two months ago and we haven't seen the issue again eversince. Also see my comment in #743, why this fixes the issue.

[danzel]

Can we have a test case that reproduces this too please?
See the spec dir.
If that's a bit tough, then at least an example in example/old-bugs.
Thanks!

[boldtrn]

The problem with tests is that they fail locally similar to Travis, so I cannot really write a test without testing it :).

I created an example.

With c7932f6 you are able to reproduce the bug. With bc40326 it's fixed.

It is a bit hard to reproduce, you need to zoom out before the cluster is fully loaded in order to trigger the removeLayers before the chunckedLoading is done. It is important to do this during the zoom.

[danzel]

I've fixed the tests, please create a test instead of the example if possible!

[boldtrn]

I've fixed the tests, please create a test instead of the example if possible!

Awesome work!

I created a test and showed that it failed and that the code change fixed the failure.

I am not sure how many markers are needed in order to provoke this issue. I settled for 1000, but I think it might be possible to further reduce it.

[danzel]

Great thanks!