Create HA1 hash by realm, username, password

    HA1 = md5(A1) = MD5(username:realm:password)

def HA1(realm, username, password, algorithm):
    """Create HA1 hash by realm, username, password

    HA1 = md5(A1) = MD5(username:realm:password)
    """
    if not realm:
        realm = u''
    return H(b":".join([username.encode('utf-8'),
                           realm.encode('utf-8'),
                           password.encode('utf-8')]), algorithm)