fix: stream! when the client adapter models response headers as lists

[pepicrft]

We are using :ex_aws_s3 package in a project with :req as a client adapter, and we were experiencing an error because the logic to obtain the content size from the response headers assumes that the header is a binary. However, :req uses a list of strings as a type.

This PR adjusts the code to handle both cases gracefully. Please, let me know if you'd like me to add an integration test where :req is used as an adapter.

[pepicrft]

Is anyone available for a review here (maybe @bernardd?)

[bernardd]

Thanks @pepicrft!