Update a `key` in the keystore.
        If the key is non-existent, it's being created

def update(self, key, value):
        """Update a `key` in the keystore.
        If the key is non-existent, it's being created
        """
        if not isinstance(value, dict):
            raise BadValueError(
                'The value {} is incorrect.'
                ' Values should be strings'.format(value))
        if key in self.data:
            v = self.get(key)
            v.update(value)
        else:
            v = value
        self.set(key, v)