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.
If `geometryName` is undefined, no attribute is considered as geometry :
https://github.com/fgravin/ol3/blob/boundedBy/src/ol/format/gml/gmlbase.js#L181

My code just tells that if the attribute name is 'boundedBy', `geometryName` should be undefined. Otherwise, it is as before. But in both cases, the node has to be read by the format as a `geometryElement` by `this.readGeometryElement()`.