Fix bounding polygon Coordinate Reference System handling for aodn/aodn-...

...portal#1406
Rather than instantiating the specific instance of WKTWriter here, would it be better to it pass via the constructor the same way you pass the parser in. This would avoid coupling the code on this particular WKT writer instance, and/or allow it to be mocked for testing etc.
 
Alternatively it only seems to be used in the toWkt(String gml) method. would it be better to just instantiate as needed here?