Use DefaultAWSCredentialsProviderChain when access-key and explicit credentials are not present

Currently faraday will only default to using the DefaultAWSCredentialsProviderChain if _no client options_ are specified (the map is empty). It's nice if faraday uses DefaultAWSCredentialsProviderChain whenever credentials can't be found, even when other options are specified.

This change means that users don't need to create their own DefaultAWSCredentialsProviderChain instance and pass it in using the `:credentials` key.