Ensure request body is always returned as bytes (#372)

This updates _get_inference_request to always return the request body as bytes, 
as per the InferenceServerClient .generate_request_body docstring.

src/python/library/tritonclient/http/_utils.py

@@ -153,4 +153,4 @@ def _get_inference_request(
         )
         return request_body, json_size
 
-    return request_body, None
+    return request_body.encode(), None