Pass list of stream values to _expand_mse_streams
    
    The original description of argument stream is inconsistent with the value
    passed to function _expand_mse_streams. The actual passed-in value is
    Modulemd.SimpleSet. To fix this inconsistency, just pass list of streams, each
    of them is a string representing a stream.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>