Fix unit test cases to work with DynamoDB Local, except two

The two test cases that still fail are probably caused by bugs in
DynamoDB Local.

1. UpdateItem calls succeed even when we update an attribute with
   an empty string. Based on the doc, empty strings and sets are
   now allowed.

2. BatchGetItem calls may return less unprocessed keys than expected.
Merge pull request #17 from Obvious/xiao-using-DynamoDB-Local-for-testing

Fix unit test cases to work with DynamoDB Local, except two