PYCBC-206: Streaming fixups

- We were using state->cur_pos which isn't actually the current position
  but rather the last position of the current child
- Added a test for this.

Change-Id: Id9756552ce8e60f77b74cbfb467b7f9a601a183d
Reviewed-on: http://review.couchbase.org/30805
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>