Add check for href value to sampledFeature insertion. If the href value is http://www.opengis.net/def/nil/OGC/0/unknown, no sampledFeature with identifier http://www.opengis.net/def/nil/OGC/0/unknown is inserted.
Merge pull request #137 from CarstenHollmann/fix/unknown_feature_insertion

Add check for href value to sampledFeature insertion.