re-implement auto-configure code for ol.format.GML

so that people don't need to specify featureNS and featureType manually, see: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/ol3-dev/VoLY-miQy_E/h3BwwpY5t0cJ
Update: I now see that I don't test the wfs case actually, but both test cases are of type '-gml'