fixed type-error that would crash the request

src/main.py

@@ -80,7 +80,7 @@ async def test_data(api_key: str, size: str) -> StreamingResponse:
             content=generate_data(size, BUFFER_SIZE),
             media_type='application/octet-stream',
             headers={
-                'Content-Length': size
+                'Content-Length': str(size)
             }
         )