Pass rootMargin as String

[danieljb]

When using IOlazy with IntersectionObserver Polyfill in Safari 11 the Polyfill’s _parseRootMargin method crashes with a TypeError e.split is not a function. (In 'e.split(/\s+/)', 'e.split' is undefined).
This is caused by IOlazy passing rootMargin as Array which can not work with the polyfill.

Please update IOlazy class to pass rootMargin as String which is – as far as I can tell – expected in the spec, too.

[cdowdy]

thanks @danieljb I dunno why I overlooked that when I merged the PR.

I'll push a fix here in a bit!

[danieljb]

Thanks for the super fast fix!