Fix: Account for requests that have already been decoded by model server

[gwang111]

Issue #, if available:
#4959

Description of changes:
Some model servers (MMS in specific) decode the request from bytes ahead of time if the content type of the request is in a known format.

In these scenarios, we should honor that logic. If the request by the time it gets to our decoder is no longer in a bytes / bytestream format then propagate it as is.

We should not get into the business of re-encoding it back into a bytes or bytestream format

In the first iteration: #4960 a regression was introduced. Fixed that here

Testing done:

tests/integ/sagemaker/serve/test_serve_mlflow_pytorch_flavor_happy.py::test_happy_pytorch_sagemaker_endpoint_with_torch_serve PASSED

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[gwang111]

@andjsmi please review this when u have the chance. Can't add you as a reviewer for some reason

Spoke offline. he is good with the change: