Fix issue #1277

Function schemaLocationAttr() in OpenLayers.Format.WFST.v1 always stores options.schema using options.featurePrefix as key in schemaLocations, and OpenLayers.Format.GML.Base's constructor always uses "feature" as a key to store featureNS in namespaces.

This fix changes OpenLayers.Format.GML.Base's behavior to use featurePrefix (when it is provided) instead of "feature", what makes schemaLocationAttr() use the right key in both objects: schemaLocations and namespaces.