TableInfo index refresh fails when secondary indexes are present

[mattsb42-aws]

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'