BUG: xlsx-renderer/Renderer mutate viewModel argument.

[Siemienik]

The renderer mutates a viewModel by adding information about the current loop states or deleting variables by using `DeleteCell. It is wrong behaviour and it shouldn't do that, because it causes next bugs, for instance, if the same VM is used twice.

Here was proposed a workaround, that makes deep copy before each render() call.

This issue will be fulfilled when calls render() function will stop mutate the ViewModel.

The quickest solution is making deep copy inside the render() function. However, it isn't a fully satisfying solution, because it will take additional time and memory. I consider making a proxy object, which read all object data from source (VM) and - if required - mutates data in a proxy.

As this bug, I estimate critical important I want to make a deep copy by using JSON.parse(JSON.stringify()) ASAP. Then I will analyze a better solution.