fix: Save deserialized credential object to Authentication member

[adebayor123]

Details

Currently in our Authentication and EnhancedAuthentication, we save credential object as serialized string values. Then, as we read the serialized string from Storage, we parse the string into a Credential object.

However, in AnonymousStorageCredentialsProvider, we overwrite the local member variable credential with the serialized string instead of the parsed object. As a result, any situation that requires reading of the credential from storage leads to: TypeError: this.credentials.expiration.getTime is not a function.

This PR removes the unnecessary step to overwrite the local member this.credential with the serialized string.

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[qhanam]

issue: We need unit tests to cover the new behavior (here and in Authentication.ts).

I think this change introduces a bug. Cover the following cases:

1. When credentials are read from storage.
2. When credentials are read from storage, and then read from the member variable.

[qhanam]

I've pushed a patch for EnhancedAuthentication.ts:

1. Add type to credentials local variable
2. Add return statement when rejecting
3. Fix expiration (to be date object) in resolved value
4. Added unit tests

[qhanam]

nit: This test will pass under certain conditions when it should fail (i.e., false negative).

Specifically, when this.credentials.expiration is undefined, and the expiration property of the value returned by AnonymousStorageCredentialsProvider is a Date object. When this happens, control flow will reach AnonymousCognitoCredentialsProvider, and the test needs a way to distinguish the value returned here from the expected value.

You can fix this by mocking the response of getCredentials, like was done in EnhancedAuthentication.test.ts here.