Return the canonical request string.

def canonical_request(self, method, path, content, timestamp):
        """Return the canonical request string."""
        request = collections.OrderedDict([
            ('Method', method.upper()),
            ('Hashed Path', path),
            ('X-Ops-Content-Hash', content),
            ('X-Ops-Timestamp', timestamp),
            ('X-Ops-UserId', self.user_id),
        ])
        return '\n'.join(['%s:%s' % (key, value)
                          for key, value in request.items()])