Fix type handling in rdfauthor popover controller

Old behaviour: When creating instances, RDFauthor empties all fields that
might contain values, except or the rdfs:type field. rdfs:type was
tested by doing a regex match on the label.

Why changed: This test might fail when someone includes an additional
label, since the test just uses any label, not every label. Moreover, the
regex might yield a false positive as it would match also sub-strings.

New behaviour: We do a simple string comparison of the URIs.