Properly unset node data

[mariusheine]

Assume there is a conditionally shown element and the user has data for that field. If this element becomes hidden the according data gets removed so the resulting object should be the same as when the user never opened the form with that element visible.

Currently this is not working, because when the element gets hidden we set the associated field in the data to undefined. The problem is that undefined is not equal to a missing field in terms of operations like isEqual from lodash.

Some sample data to visualize the difference:

element 'b' shown with value	element 'b' hidden afterwards	element 'b' never was displayed
{ a: 10, b: 'shown' }	{ a: 10, b: undefined }	{ a: 10 }