Possible Web Map Service WMS interface for GeoAxes

See IPython Notebook example:
http://nbviewer.ipython.org/urls/gist.githubusercontent.com/isedwards/10992191/raw/c6ca17b13cf853d2031029e86d27fd81bec07292/wms.ipynb?create=1

Is the use of `isinstance` appropriate? And would the test mock the WebMapService object?