Fix issue where groups iteration in wizard step failed with Chameleon

Hi, has anyone tried c.z3cform.wizard with Chameleon (via five.pt) recently?

This fixes an issue, which we had with Chameleon 2.14.

This could be repeated simply by adding and configuring Chameleon support (via five.pt). I could do this, but how would you like it? Can I just add Chameleon into existing test suites and fix doctest to test against Chameleon produced HTML? (Without this patch, a couple of c.z3cform.wizard tests will give Chameleon error, and with this patch they would just fail because the HTML output is a bit different.)