actually fix unicode-path-query exception

I mistakenly assumed that the value sent to _regexp from SQLite would be a str
object. It's a buffer object, of course. This change explicitly converts to a
str before doing the decoding.

beets/util/__init__.py

@@ -473,6 +473,8 @@ def as_string(value):
     """
     if value is None:
         return u''
+    elif isinstance(value, buffer):
+        return str(value).decode('utf8', 'ignore')
     elif isinstance(value, str):
         return value.decode('utf8', 'ignore')
     else: