tests: deserialized AWS signer for IAM auth fails

[ArzelaAscoIi]

Related Issues

• fixes #issue-number

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• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
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[anakin87]

I've copied here my Slack message. 👇

I found 2 different errors:

• as you pointed out, SentenceWindowRetrieval deserialization is wrong -> should use doc_store_class.from_dict instead of default_from_dict
• OpenSearchDocumentStore from_dict method tries to deserialize _http_auth, but to_dict stores this info in http_auth

[anakin87]

The issue was fixed (tests are passing).

@ArzelaAscoIi @FHardow feel free to close this PR if everything is OK.