Mock up requests.head(..) response on local file.

def head_on_file(self, file):
        """Mock up requests.head(..) response on local file."""
        response = HeadResponse()
        if (not os.path.isfile(file)):
            response.status_code = '404'
        else:
            response.status_code = '200'
            response.headers[
                'last-modified'] = datetime_to_str(os.path.getmtime(file))
            response.headers['content-length'] = os.path.getsize(file)
        return(response)