One fix for bug #1154: webapi GETs with a 'Range' header broke new-downloader.

One fix for bug #1154: webapi GETs with a 'Range' header broke new-downloader.

The Range header causes n.read() to be called with an offset= of type 'long',
which eventually got used in a Spans/DataSpans object's __len__ method.
Apparently python doesn't permit __len__() to return longs, only ints.
Rewrote Spans/DataSpans to use s.len() instead of len(s) aka s.__len__() .
Added a test in test_download. Note that test_web didn't catch this because
it uses mock FileNodes for speed: it's probably time to rewrite that.

There is still an unresolved error-recovery problem in #1154, so I'm not
closing the ticket quite yet.