Update shapely usage to avoid deprecation

[frafra]

Work based on PR #272.

[frafra]

The only potential problem I see, is that the error message for an invalid geometry is a bit too generic.
shape('{"Type":"Bad_GeoJSON","a":2}') would show AttributeError: 'NoneType' object has no attribute 'lower', which is not so great. An alternative would be to catch AttributeError as an exception and write something like Invalid geometry.

[frafra]

Instead of changing the tests, I changed the error handling in the code, and wrote a meaningful error message.

That seems good enough for me :)

[mutantsan]

Thanks, @frafra !
I couldn't find the time to fix the tests. I'm glad that you found the issue.