'Test renaming keyword with rename_keyword.'

def test_rename_keyword(self):

    header = fits.Header([('A', 'B', 'C'), ('D', 'E', 'F')])
    header.rename_keyword('A', 'B')
    assert ('A' not in header)
    assert ('B' in header)
    assert (header[0] == 'B')
    assert (header['B'] == 'B')
    assert (header.comments['B'] == 'C')
