Default handler for Expect header.

    Just send "100 Continue" to client.
    raise HTTPExpectationFailed if value of header is not "100-continue"

async def _default_expect_handler(request: Request) -> None:
    """Default handler for Expect header.

    Just send "100 Continue" to client.
    raise HTTPExpectationFailed if value of header is not "100-continue"
    """
    expect = request.headers.get(hdrs.EXPECT)
    if request.version == HttpVersion11:
        if expect.lower() == "100-continue":
            await request.writer.write(b"HTTP/1.1 100 Continue\r\n\r\n")
        else:
            raise HTTPExpectationFailed(text="Unknown Expect: %s" % expect)