StreamListener.filter(): follow argument doesn't accept integers for user ids

[dannguyen]

This is for tweepy 3.5.0, Python 3.5.2

In regards to what's shown in this example: #827

  myStream.filter(follow=[2211149702])

That doesn't work in tweepy 3.5. Instead, you get this error message:

 TypeError: sequence item 0: expected str instance, int found

The problematic line is in tweepy/streaming.py:

          428         if follow:
   --> 429             self.body['follow'] = u','.join(follow).encode(encoding)

Expected behavior: the follow argument can handle integers or strings; in the former case, the int is stringified.