Improve download mechanism when Range HTTP header is ignored

Range HTTP header may be ignored/unsupported by a webserver (e.g. [`extractor/scivee.py`](https://github.com/rg3/youtube-dl/blob/master/youtube_dl/extractor/scivee.py), [`extractor/bambuser.py`](https://github.com/rg3/youtube-dl/blob/master/youtube_dl/extractor/bambuser.py)). However, for a test we still would like to download just a piece of a file.
This PR works around *ignored-Range-issue* by additional checks and block size calculation.