Register a URL prefix to passthru any non-matching mock requests to.

        For example, to allow any request to 'https://example.com', but require
        mocks for the remainder, you would add the prefix as so:

        >>> responses.add_passthru('https://example.com')

def add_passthru(self, prefix):
        """
        Register a URL prefix to passthru any non-matching mock requests to.

        For example, to allow any request to 'https://example.com', but require
        mocks for the remainder, you would add the prefix as so:

        >>> responses.add_passthru('https://example.com')
        """
        if _has_unicode(prefix):
            prefix = _clean_unicode(prefix)
        self.passthru_prefixes += (prefix,)