Modal: pressing escape key should give a dismiss reason

[tkrotoff]

Small remark.

At modal.js#L203 no reason is given:

$modalStack.dismiss(modal.key);

It should be symmetric with modal.js#L313:

$modalStack.dismiss(modal.key, 'backdrop click');

[bekos]

@tkrotoff Nice catch. What about a PR? It should be relatively easy.