Using a chart inside iron-collapse causes unecessary refresh

[plequang]

When using an nvd3-pie or nvd3-donut element within an iron-collapse element, the chart element receive iron-resize events when we mouse hover the chart.

This is because iron-collapse fires iron-resize events when it receives transitionend events.

Donut and pie charts use css transitions to change the opacity of the chart slices, causing a transitionend event to bubble up to iron-collapse.

This is not a nvd3-elements issue, but maybe nvd3-elements could provide a workaround to avoid this (by stopping transitionend events propagation).

I will submit a PR, let me know if you think it's relevant.

[saeidzebardast]

Thanks for PR. I've merged it.