Fixed zoomToShowLayer() markers disappearing bug

[z3ut]

Hello,
this will fix #739

Timeout allows to handle map movestart event before checking it execution, so layer parent won't spiderfy when move start.

Fixed example from issue http://codepen.io/anon/pen/wgBBJV

[danzel]

I'm not sure this is the right way to solve this issue.
Is there a tidier way to detect this?

[z3ut]

What use case for movestart? Without movestart event it seems to work fine.

Original issue occurs when marker can be displayed on next zoom level and his parent is in map view. It reproduced here http://codepen.io/anon/pen/rjVgVw

I added some tests for zoomToShowLayer function (Well, at least I tried). It covers the issue, but it looks like the problem has different behaviour in PhantomJS and real browsers (Chrome 55, Firefox 50). After zoom in browser marker._icon == null, but not for tests. I checked with 200px*200px map in browser (like for tests), same result. More likely it's my fault, but maybe someone saw similar before.

Am I missing the function logics and some use cases, or movestart event can be removed?

[danzel]

I've retested the original issue, it looks fine with this fix (#286).
Going to merge and do a bit more testing. Thanks!