Fixed signal.get_window with unicode window name

This minor fix allows unicode objects to be used in scipy.signal.get_window.

This code does not fail anymore (python2):
    from scipy.signal import get_window
    get_window(u'blackman', 32) # This failed because u'blackman' is not a string on python2.

This code does not fail anymore (python2):
    from __future__ import unicode_literals
    from scipy.signal import get_window
    get_window('blackman', 32) # This failed because u'blackman' is unicode