Fix backwards compatibility for .write_bytes() (#11336)

Author: Dreamsorcerer

aiohttp/client_reqrep.py

@@ -1334,7 +1334,7 @@ async def write_bytes(
         self,
         writer: AbstractStreamWriter,
         conn: "Connection",
-        content_length: Optional[int],
+        content_length: Optional[int] = None,
     ) -> None:
         """
         Write the request body to the connection stream.