Stringify the Content-Length header

Recent versions of requests no longer accept integer header values.

See https://github.com/kennethreitz/requests/issues/3477

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>

musicbrainzngs/musicbrainz.py

@@ -549,7 +549,7 @@ def _mb_request(path, method='GET', auth_required=AUTH_NO,
     else:
         # Explicitly indicate zero content length if no request data
         # will be sent (avoids HTTP 411 error).
-        headers['Content-Length'] = 0
+        headers['Content-Length'] = '0'
 
     # Convert args from a dictionary to a list of tuples
     # so that the ordering of elements is stable for easy