Re-fixes range restriction code

Unfortunately you broke the fix I've sent earlier in https://github.com/arskom/spyne/pull/227 

ProtocolBase.to_string needs a type as argument to determine how that type should be turned into a string.

I modified the unit tests so this code path is not skipped anymore and the error will show. I didn't create a specific test for this since I couldn't determine where the test should go.