[BUGFIX] Eliminate side-effects from ForViewHelper

[s2b]

The previous implementation overwrote template variables with the same name as the specified ViewHelper parameters "as", "key", or "iteration". With the new implementation, the variable state before the ViewHelper call gets restored once the ViewHelper call is completed.

[s2b]

Just noticed that this would break the following hack/work-around:

<f:for each="{items}" as="item">
  <f:variable name="lastItem" value="{item}" />
</f:for>

Maybe we need some kind of merge logic here?

[s2b]

The new version of this patch should resolve this problem.

[s2b]

I think that there is still an edge case here:

<f:variable name="lastItem" value="fallback" />
<f:for each="{items}" as="item">
  <f:variable name="lastItem" value="{item}" />
</f:for>

In this case, lastItem would always be fallback because the variable gets restored to its initial state after the loop.

Edit: Checked it with tests, this is in fact NOT the case, because only variables defined by f:for will be reset.