Wrap transform methods of Robinson projection to avoid passing NaNs.

Attempts to workaround https://github.com/SciTools/cartopy/issues/232

Notes:
  * found that "transform_points" (plural!) has the same problem
  * scanned CRS methods for any more that might be affected (+ didn't find any)
  * I have not looked "under the hood", so don't know whether this is really in PROJ4 or only the Python wrapper.  

It would probably be a useful follow-on for someone to identify the actual source of the problem and push back on the relevant  project.  You might also want to require that as part of this work.