BUGFIX: validated step will not be validated again

[MarcoPNS]

Previously, an already validated step was not validated again when resending. This happens now.

Co-Authored-By: @arsors

[mficzel]

Thanks for the pr, i understand the issue and agree that it should be solved.

However i somehow would prefer a solution that not specifically covers the __prev case
but also allows to render a navigation with multiple steps alla 1 - 2 - 3 ...

I suggest to use the name of the button that is clicked to separate between submits may cause a commit to the state and those that do not (steps backwards). Maybe __targetNoCommit alternatively to __target but i am not sure about the naming.

[mficzel]

@MarcoPNS i build #72 upon your changes. Instead of a __prev key it uses an ! as prefix for the __target to identify the back navigation. I prefer this as this would work even when a custom form navigation with multiple keys is rendered.

Could you try this and give me feedback wether this works in your case.