http.go
func (req *Request) MayContinue() bool {
	return bytes.Equal(req.Header.peek(strExpect), str100Continue)
}
func (req *Request) MayContinue() bool {
	return bytes.Equal(req.Header.peek(strExpect), str100Continue)
}
// MayContinue returns true if the request contains
// 'Expect: 100-continue' header.
//
// The caller must do one of the following actions if MayContinue returns true:
//
//     - Either send StatusExpectationFailed response if request headers don't
//       satisfy the caller.
//     - Or send StatusContinue response before reading request body
//       with ContinueReadBody.
//     - Or close the connection.