Fix "RuntimeWarning: Argument <type 'str'… …> is not an unicode object"

For files with no tags I got the RuntimeWarning:
/usr/local/lib/python2.7/dist-packages/beets-1.1.0_beta.2-py2.7.egg/beets/autotag/match.py:104: RuntimeWarning: Argument <type 'str'> is not an unicode object. Passing an encoded string will likely have unexpected results.
  unidecode(str1)

Adding some debug output to _string_dist_basic in match.py showed the following:
[ ... ]
STR1 <type 'unicode'> 
STR2 <type 'unicode'> cavalier eternal
STR1 <type 'str'> 
STR2 <type 'unicode'> against me!
STR1 <type 'str'> 
STR2 <type 'unicode'> as the eternal cowboy
STR1 <type 'unicode'> 
STR2 <type 'unicode'> slurring the rhythms
[ ... ]

This commit probably doesn't fix the root cause, but anyways...