Fix MostRecentProvider.encryption_materials() caching bug

[mattsb42-aws]

Description of changes:
A customer encountered a bug where MostRecentProvider was not actually using cached results. Upon investigation, this uncovered this bug where the provider was returning invalid results once the cache was populated.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[johnwalker]

Working on understanding the fix here. It seems that the original logic was this: check if ttl_action is TtlActions.LIVE, and if so return the given provider from the cache. Also, in this case return a provider instead of encryption materials like in the other cases.

If it failed, set the ttl_action to EXPIRED and pull the provider from the local cache if thats allowed. If it's not allowed, miss the cache.

Is it the case that this fixes the return types of the method to be consistent? And then, was there a problem with the cache returning None for its values?

[mattsb42-aws]

Is it the case that this fixes the return types of the method to be consistent?

Yes. What used to happen was that the return type from encryption_materials() would be different if there was cache miss (EncryptionMaterials) than if there was a cache hit (CryptographicMaterialsProvider).

This test[1] failing was what identified this issue.

[1] https://github.com/aws/aws-dynamodb-encryption-python/pull/102/files#diff-6420d750c8994c892392e8d4caaa35fbR81

[johnwalker]

Thanks! Good find