Read request body if present.

        Returns bytes object with full request content.

async def read(self) -> bytes:
        """Read request body if present.

        Returns bytes object with full request content.
        """
        if self._read_bytes is None:
            body = bytearray()
            while True:
                chunk = await self._payload.readany()
                body.extend(chunk)
                if self._client_max_size:
                    body_size = len(body)
                    if body_size >= self._client_max_size:
                        raise HTTPRequestEntityTooLarge(
                            max_size=self._client_max_size,
                            actual_size=body_size
                        )
                if not chunk:
                    break
            self._read_bytes = bytes(body)
        return self._read_bytes