Configuration test xml

follow-up for #103 - there are some issues with xml that i hope somebody can help me with.

until #103 is merged, this will also show all changes of #103
nope. ``fixXmlConfig('vary')`` would turn ``vary`` into ``varys`` (or whatever other plural you define), so it would even make thing worse by breaking YAML configs as well.

I think the current setup works for the case of a single ``<vary>`` child because of the ``ifString()`` normalization exploding the string.