Fix tweepy.API.update_with_media

If filename is an instance of `unicode',
L.76 `` 'Content-Disposition: form-data; name="%s"; filename="%s"' % (form_field, filename) ''
must be unicode string.

Because of it, L. 803 `` body = '\r\n'.join(body) '' fails.

Therefore, I made `filename' designed to be percent-encoded str object
before formatting.