Element boundedBy must not be set as geometry field on GML reading

Right now, during a GML feature reading, if the feature contains no geometry, the boundedBy attribute is considered as the geometry attribute of the feature, while it is just an extent. This rises an exception.

This PR proposes to skip boundedBy attribute when we're trying to attach the geometry attribute to the feature.
I think we should use ol.xml.getLocalName everywhere here and only use it once?