[PR #9693/dd0b6e37 backport][3.11] Avoid duplicate content_length fetch to prepare web response headers (#9703)

Co-authored-by: J. Nick Koston <[email protected]>

Author: patchback[bot]

aiohttp/web_response.py

@@ -483,7 +483,7 @@ async def _prepare_headers(self) -> None:
                 headers[hdrs.TRANSFER_ENCODING] = "chunked"
             if hdrs.CONTENT_LENGTH in headers:
                 del headers[hdrs.CONTENT_LENGTH]
-        elif self._length_check:
+        elif self._length_check:  # Disabled for WebSockets
             writer.length = self.content_length
             if writer.length is None:
                 if version >= HttpVersion11:
@@ -504,7 +504,7 @@ async def _prepare_headers(self) -> None:
             # https://datatracker.ietf.org/doc/html/rfc9112#section-6.1-13
             if hdrs.TRANSFER_ENCODING in headers:
                 del headers[hdrs.TRANSFER_ENCODING]
-        elif self.content_length != 0:
+        elif (writer.length if self._length_check else self.content_length) != 0:
             # https://www.rfc-editor.org/rfc/rfc9110#section-8.3-5
             headers.setdefault(hdrs.CONTENT_TYPE, "application/octet-stream")
         headers.setdefault(hdrs.DATE, rfc822_formatted_time())