In-place data changes lead to non-reactivity in route data context

[tmeasday]

See iron-meteor/iron-router#667

Reproduction: https://github.com:tmeasday/IR693

The issue is due to https://github.com/EventedMind/blaze-layout/blob/master/layout.js#L144

[tmeasday]

@cmather - I foolishly didn't record what we decided here. Was it that we'd make setData take a copy to compare with?