Prepare Content-Length header based on request method and body

def prepare_content_length(self, body):
        """Prepare Content-Length header based on request method and body"""
        if body is not None:
            length = super_len(body)
            if length:
                # If length exists, set it. Otherwise, we fallback
                # to Transfer-Encoding: chunked.
                self.headers['Content-Length'] = builtin_str(length)
        elif self.method not in ('GET', 'HEAD') and self.headers.get('Content-Length') is None:
            # Set Content-Length to 0 for methods that can have a body
            # but don't provide one. (i.e. not GET or HEAD)
            self.headers['Content-Length'] = '0'