def coerce_url(url, default_schema='http'):
    """
    >>> coerce_url('https://blog.guyskk.com/feed.xml')
    'https://blog.guyskk.com/feed.xml'
    >>> coerce_url('blog.guyskk.com/feed.xml')
    'http://blog.guyskk.com/feed.xml'
    >>> coerce_url('feed://blog.guyskk.com/feed.xml')
    'http://blog.guyskk.com/feed.xml'
    """
    url = url.strip()
    if url.startswith("feed://"):
        return "{}://{}".format(default_schema, url[7:])
    if "://" not in url:
        return "{}://{}".format(default_schema, url)
    return url