Use `react/lib/Object.assign` instead of `react/lib/merge` and `react/lib/copyProperties`

It removes the following warnings:
- `react/lib/merge` has been deprecated and will be removed in the next version of React. All uses can be replaced with `Object.assign({}, a, b)` or `_.extend({}, a, b)`. 
- `react/lib/copyProperties` has been deprecated and will be removed in the next version of React. All uses can be replaced with `Object.assign(obj, a, b, ...)` or` _.extend(obj, a, b, ...)`.