XML based mappers don't serialize to string properly

[azaroth42]

>>> d = cfgs.external['viaf']['datacache']['8612829']
>>> r = cfgs.external['viaf']['mapper'].transform(d, 'Person') 
>>> type(r['data']['_label'])
<class 'lxml.etree._ElementUnicodeResult'>

And thus any test based on the transformed version, rather than the version that has been serialized to cache and then retrieved, will be weird.