Check if channel is valid.

def check_valid_channel(self,
                            channel,
                            conda_url='https://conda.anaconda.org'):
        """Check if channel is valid."""
        if channel.startswith('https://') or channel.startswith('http://'):
            url = channel
        else:
            url = "{0}/{1}".format(conda_url, channel)

        if url[-1] == '/':
            url = url[:-1]
        plat = self.conda_platform()
        repodata_url = "{0}/{1}/{2}".format(url, plat, 'repodata.json')
        worker = self.download_is_valid_url(repodata_url)
        worker.url = url
        return worker