Avoid downloading when Content-Length is bigger than MAX_SIZE

Continued from 60164a9. With this, we don't need to download anything
if we are dealing with a nice server that properly sets Content-Length.

Unfortunately, we still have to check during download as python-requests
does not abstract that bit away.
Merge pull request #587 from jleclanche/master

Avoid downloading when Content-Length is bigger than MAX_SIZE