Prototype Pollution in the merge and clone helper methods

Impact

Using merge and clone helper methods in the src/core/util.ts module will have prototype pollution. It will affect the popular data visualization library Apache ECharts, which is using and exported these two methods directly.

Patches

It has been patched in https://github.com/ecomfe/zrender/pull/826.
Users should update zrender to 5.2.1. and update echarts to 5.2.1 if project is using echarts.