FIX: make_inverse_operator for free orientation

The way I used to use make_inverse_operator for free orientation now raises an error. 

    ValueError: Cannot make inverse with fixed-orientation forward solution unless fixed is True.

Did I do that in a wrong way (make_inverse_operator with fixed=False, loose=None)? If not, something seems to be wrong with the error conditions (raising an error `if loose is None and not fixed`). I attempted to fix them for my case. Now a test fails because an error is not raised, but that is how I would have made a free orientation inverse operator (https://github.com/mne-tools/mne-python/blob/master/mne/minimum_norm/tests/test_inverse.py#L172):

    fwd_op = read_forward_solution(fname_fwd, surf_ori=True)
    ...
    assert_raises(ValueError, make_inverse_operator, evoked.info, fwd_op,
                  noise_cov, depth=0.8, loose=None)