TableInfo index refresh fails when secondary indexes are present #75
Description
We don't actually use the secondary indexes for anything, but TableInfo is opportunistically collecting them when refreshing index data. Unfortunately, TableIndex objects are not hashable, we are currently storing them in a set, and apparently this slipped through the cracks in testing...so that fails.
>>> table_info.refresh_indexed_attributes(client)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/bullocm/git/aws-dynamodb-encryption-python/.tox/py36-local-fast/lib/python3.6/site-packages/dynamodb_encryption_sdk/structures.py", line 385, in refresh_indexed_attributes
self._secondary_indexes.add(TableIndex.from_key_schema(index['KeySchema']))
TypeError: unhashable type: 'TableIndex'