make 'a' and 'b' coefficients atleast_1d array in filtfilt

The following crashes:

    import numpy as np
    from scipy.signal import filtfilt
    x = np.random.randn(100)
    y = filtfilt(np.ones(5)/5, 1, x)  # filtfilt(b, a, x)

The problem is in lines 1931 and 1932 of the filfilt function:

    b = np.asarray(b)
    a = np.asarray(a)

These lines should be replaced by:

    b = np.atleast_1d(b)
    a = np.atleast_1d(a)

In order to the line 1935 to work properly:

    ntaps = max(len(a), len(b))