Use schema from global indexes as well as local

This fixed ```AWS::DynamoDB::Errors::ValidationException: Validation error detected: The provided starting key is invalid``` for me.

P.S. As you can see this is my third PR, I just can't stop, but this is probably last one for a now.
Please feel free to disregard my changes or provide any suggestions if you don't think that anything is incorrect.

Thanks :sparkles: